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
96a01fa046