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

llvm-svn: 237833