[libFuzzer] fix a warning from Wunreachable-code-loop-increment reported by...

[libFuzzer] fix a warning from Wunreachable-code-loop-increment reported by Christian Holler. This also fixes a logical bug, which however does not affect the libFuzzer's ability too much (I wasn't able to create a differentiating test)

llvm-svn: 303087