[GWP-ASan] Handle wild touches of the guarded pool.

AllocMeta could be null when returned from __gwp_asan_get_metadata() for
a bad access into the GuardedPagePool that was never allocated.
Currently, then we dereference the null pointer, oops.

Hoist the check up and print a message (only once in recoverable mode)
about the bad memory access.

Reviewed By: fmayer

Differential Revision: https://reviews.llvm.org/D144973