[BitcodeReader] Validate Strtab before accessing.

This fixes a crash with invalid bitcode files that have records
referencing names in Strtab, but Strtab is not present or the index is
out-of-bounds.

This fixes the following clusterfuzz issue:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29895

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D95554