[libfuzzer] moving is_ascii handler inside mutation dispatcher.

Summary: It also fixes a bug, when first random might not be ascii.

Differential Revision: http://reviews.llvm.org/D21573

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273611 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed