fuzzer_mutate should _technically_ be unsafe

[Manishearth]

libfuzzer/src/lib.rs

Line 601 in 719e4ef

pub fn fuzzer_mutate(data: &mut [u8], size: usize, max_size: usize) -> usize {

Technically, this should only be called within a fuzz loop

https://github.com/llvm/llvm-project/blob/b0a64cf46adcb6da2cef28bac25f910077277923/compiler-rt/lib/fuzzer/FuzzerLoop.cpp#L942-L943

(otherwise fuzzer::F is undefined)

In practice, I don't really think this is an issue, but we should perhaps try and find a way to assert that (or mark it unsafe)