[FileCheck] - Fix possible buffer out of bounds access when parsing --check-prefix.

[FileCheck] - Fix possible buffer out of bounds access when parsing --check-prefix.

FileCheck tool crashes when trying to parse --check-prefix argument if there is no any
data after it.

For example test like following would crash if there are no symbols and no EOL mark after `boom`:

# REQUIRES: x86
# RUN: <skipped few lines>
# RUN: llvm-readobj -t %t | FileCheck %s --check-prefix=boom

Patch fixes the issue.

Differential revision: https://reviews.llvm.org/D42057

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322536 91177308-0d34-0410-b5e6-96231b3b80d8