[libFuzzer] change the default max_len from 64 to 4096. This will affect cases where...

[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.

Change-Id: Ia396f702ec7c8a0c637837240282f85574e1af3a
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305521 91177308-0d34-0410-b5e6-96231b3b80d8