Allow corpus removal during main fuzz loop

[hgarrereyn]

While working on a fuzzer I was trying to implement a stage that would sometimes remove elements from the corpus. It is theoretically possible except I kept hitting this error in StdFuzzer::fuzz_one where it expected the corpus element to exist after the completion of all the stages.

The patch here just allows fuzzing to continue if the corpus entry no longer exists, rather than throwing an error as it did before.

[domenukk]

thread 'rustc' panicked at compiler/rustc_infer/src/infer/generalize.rs:28:40:
called `Result::unwrap_err()` on an `Ok` value: ?11t

oO

Anyway @addisoncrump could you take a look you know more about side effects of removing testcases I guess

[addisoncrump]

Hm, the error associated with fetching the testcase should probably be checked. Also, idx should be updated if the testcase doesn't exist, because there are other stages which expect testcases to exist.

[domenukk]

What should we do here?

[domenukk]

Merging this for now, feel free to fix @addisoncrump