In MemoryBuffer::getOpenFile() make sure that the buffer is null-terminated if

the caller requested a null-terminated one.

When mapping the file there could be a racing issue that resulted in the file being larger
than the FileSize passed by the caller. We already have an assertion
for this in MemoryBuffer::init() but have a runtime guarantee that
the buffer will be null-terminated, so do a copy that adds a null-terminator.

Protects against crash of rdar://11161822.

llvm-svn: 154082