[llvm] [MC] AsmLexer invalid read fix. (PR #154972)

[Szymon Piotr Milczek via llvm-commits]

smilczek wrote:

Note self: need to create a binary file which will end with 0x20 (space character) and another file that will contain the commands to run the test. Possibly put `assert(CurPtr <= CurBuf.end())` in `getNextChar()` to force a failure in case of invalid read.

https://github.com/llvm/llvm-project/pull/154972