AsmParser: Require a terminating null character when creating memory buffer.

AsmParser: Require a terminating null character when creating memory buffer.

This commit modifies the memory buffer creation in the AsmParser library so
that it requires a terminating null character. The LLLexer in the AsmParser
library checks for EOF only when it sees a null character, thus it would
be best to require it when creating a memory buffer so that the memory
buffer constructor can verify that a terminating null character is indeed
present.

Reviewers: Duncan P. N. Exon Smith, Matthias Braun

Differential Revision: http://reviews.llvm.org/D9883

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237833 91177308-0d34-0410-b5e6-96231b3b80d8