Lex: Never overflow the file in HeaderMap::lookupFilename()

If a header map file is corrupt, the strings in the string table may not
be null-terminated.  The logic here previously relied on `MemoryBuffer`
always being null-terminated, but this isn't actually guaranteed by the
class AFAICT.  Moreover, we're seeing a lot of crash traces at calls to
`strlen()` inside of `lookupFilename()`, so something is going wrong
there.

Instead, use `strnlen()` to get the length, and check for corruption.

Also remove code paths that could call `StringRef(nullptr)`.  r261459
made these rather obvious (although they'd been there all along).

llvm-svn: 261461