[MC] AsmLexer assert buffer is null-terminated at CurBuf.end() (#154972)
AsmLexer expects the buffer it's provided for lexing to be
NULL-terminated, where the NULL terminator is pointed to by
`CurBuf.end()`. However, this expectation isn't explicitly stated
anywhere.
This commit adds a couple of comments as well as an assert as means of
documenting this expectation.