chore: Add fuzz testing creating dirs and writting files.

[ajnavarro]

Panic caught:

fuzz: elapsed: 0s, gathering baseline coverage: 0/254 completed
fuzz: elapsed: 1s, gathering baseline coverage: 254/254 completed, now fuzzing with 12 workers
fuzz: elapsed: 3s, execs: 13770 (4588/sec), new interesting: 29 (total: 283)
fuzz: elapsed: 6s, execs: 26422 (4214/sec), new interesting: 41 (total: 295)
fuzz: elapsed: 9s, execs: 44758 (6120/sec), new interesting: 53 (total: 307)
fuzz: elapsed: 12s, execs: 60665 (5303/sec), new interesting: 60 (total: 314)
fuzz: elapsed: 15s, execs: 74012 (4448/sec), new interesting: 64 (total: 318)
fuzz: elapsed: 18s, execs: 92608 (6200/sec), new interesting: 66 (total: 320)
fuzz: elapsed: 21s, execs: 105352 (4247/sec), new interesting: 71 (total: 325)
fuzz: elapsed: 24s, execs: 115204 (3284/sec), new interesting: 74 (total: 328)
fuzz: elapsed: 27s, execs: 124056 (2947/sec), new interesting: 78 (total: 332)
fuzz: elapsed: 30s, execs: 134419 (3458/sec), new interesting: 86 (total: 340)
fuzz: elapsed: 33s, execs: 145090 (3557/sec), new interesting: 89 (total: 343)
fuzz: elapsed: 36s, execs: 157856 (4255/sec), new interesting: 92 (total: 346)
fuzz: minimizing 208-byte failing input file
fuzz: elapsed: 38s, minimizing
--- FAIL: FuzzMkdirAndWriteConcurrently (38.09s)
    --- FAIL: FuzzMkdirAndWriteConcurrently (0.00s)
        testing.go:1349: panic: runtime error: invalid memory address or nil pointer dereference
            goroutine 12871 [running]:
            runtime/debug.Stack()
                /usr/local/go/src/runtime/debug/stack.go:24 +0x90
            testing.tRunner.func1()
                /usr/local/go/src/testing/testing.go:1349 +0x1f2
            panic({0xa6ec60, 0xefe300})
                /usr/local/go/src/runtime/panic.go:838 +0x207
            github.com/ipfs/go-unixfs/hamt.(*childer).has(...)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:886
            github.com/ipfs/go-unixfs/hamt.(*Shard).swapValue(0xc039753730, {0xbc1440, 0xc00012e000}, 0xc032561348, {0xc000028d69, 0x14}, 0xc008ccd710)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:623 +0xa3
            github.com/ipfs/go-unixfs/hamt.(*Shard).swapValue(0xc0397536c0, {0xbc1440, 0xc00012e000}, 0xc032561348, {0xc000028d69, 0x14}, 0xc008ccd710)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:689 +0x745
            github.com/ipfs/go-unixfs/hamt.(*Shard).SetLink(0xc0397536c0, {0xbc1440, 0xc00012e000}, {0xc000028d69, 0x14}, 0xc0004af9b0)
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/hamt/hamt.go:248 +0x1f7
            github.com/ipfs/go-unixfs/io.(*BasicDirectory).switchToSharding(0xc00015ea20, {0xbc1440, 0xc00012e000})
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/io/directory.go:337 +0x2db
            github.com/ipfs/go-unixfs/io.(*DynamicDirectory).AddChild(0xc000305db0, {0xbc1440, 0xc00012e000}, {0xc00bbf0b49, 0x2}, {0xbc32b8, 0xc039751c80})
                /home/ajnavarro/go/pkg/mod/github.com/ipfs/go-unixfs@v0.4.0/io/directory.go:569 +0x113
            github.com/ipfs/go-mfs.(*Directory).Mkdir(0xc0001b8800, {0xc00bbf0b49, 0x2})
                /home/ajnavarro/workspace/go-mfs/dir.go:316 +0x516
            github.com/ipfs/go-mfs.Mkdir(0xc000305d90, {0xc00bbf0b49, 0x2}, {0x0?, 0x0?, {0x0?, 0x0?}})
                /home/ajnavarro/workspace/go-mfs/ops.go:175 +0x31f
            github.com/ipfs/go-mfs.FuzzMkdirAndWriteConcurrently.func1(0xc0396fb520, 0xd0?, 0x17?, {0xc00bbf0b49, 0x2}, {0xc00bbf0b71, 0x1}, {0xc00dd5b940, 0x1, 0x40})
                /home/ajnavarro/workspace/go-mfs/mfs_test.go:1540 +0x8d
            reflect.Value.call({0xa75ae0?, 0xc000305dc0?, 0x13?}, {0xaf8ffc, 0x4}, {0xc0396135c0, 0x6, 0x8?})
                /usr/local/go/src/reflect/value.go:556 +0x845
            reflect.Value.Call({0xa75ae0?, 0xc000305dc0?, 0x514?}, {0xc0396135c0, 0x6, 0x8})
                /usr/local/go/src/reflect/value.go:339 +0xbf
            testing.(*F).Fuzz.func1.1(0x0?)
                /usr/local/go/src/testing/fuzz.go:337 +0x231
            testing.tRunner(0xc0396fb520, 0xc03921d320)
                /usr/local/go/src/testing/testing.go:1439 +0x102
            created by testing.(*F).Fuzz.func1
                /usr/local/go/src/testing/fuzz.go:324 +0x5b8
            
    

Heavily related to ipfs/kubo#9063, but it is not the same panic.

Signed-off-by: Antonio Navarro Perez antnavper@gmail.com

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[aschmahmann]

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