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

[grimar]

Summary

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

For example following test would crash if there is no symbols (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.

[grimar]

• Added patch context.

[arsenm]

test/FileCheck/check-empty2.txt

2–4

Isn't this a non-empty file from the comments?

[grimar]

• Updated comment.

test/FileCheck/check-empty2.txt

2–4

Clarified the comment.

[arsenm]

LGTM

test/FileCheck/check-empty2.txt

+; Check that tool does not crash for empty file.
+
+; RUN: not FileCheck -input-file %s %s -check-prefix=A
 No newline at end of file

utils/FileCheck/FileCheck.cpp

 }
 
 static Check::CheckType FindCheckType(StringRef Buffer, StringRef Prefix) {
+  if (Buffer.size() <= Prefix.size())
+    return Check::CheckNone;
+
   char NextChar = Buffer[Prefix.size()];
 
   // Verify that the : is present after the prefix.