Fix Nil Pointer Dereference on HTTP Client errors

[justabaka]

I just noticed that #145 introduces sefgaults due to nil pointer dereference on HTTP Client errors (e.g., when there's a DNS lookup problem, the server address/port is wrong etc etc). I think we should check err before trying to access resp which, obviously, can be nil.

[michaelklishin]

Thank you!

[JaSei]

Hi, this fix isn't released (tagged)
please can you release it?
thanks

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x1197a47]

goroutine 1 [running]:
github.com/michaelklishin/rabbit-hole/v2.executeRequest(0xc0000a1980, 0xc0001c4200, 0x26, 0xd, 0x56783a6c64696573)
	/home/seidl/GoWorkspace/pkg/mod/github.com/michaelklishin/rabbit-hole/v2@v2.1.0/client.go:114 +0x87
github.com/michaelklishin/rabbit-hole/v2.executeAndParseRequest(0xc0000a1980, 0xc0001c4200, 0x121a360, 0xc0002045c0, 0x0, 0x0)
	/home/seidl/GoWorkspace/pkg/mod/github.com/michaelklishin/rabbit-hole/v2@v2.1.0/client.go:122 +0x9c
github.com/michaelklishin/rabbit-hole/v2.(*Client).ListUsers(0xc0000a1980, 0x32, 0x1460f08, 0x5, 0x146a768, 0x11)
	/home/seidl/GoWorkspace/pkg/mod/github.com/michaelklishin/rabbit-hole/v2@v2.1.0/users.go:73 +0xec