[libFuzzer] change the default max_len from 64 to 4096. This will affect cases where libFuzzer is run w/o initial corpus or with a corpus of very small items.

git-svn-id: svn://svn.chromium.org/llvm-project/llvm/trunk/lib/Fuzzer@305521 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
9 files changed