[sanitizer] Fix Fuchsia ReadBinaryName not to crash when uninitialized

If the sanitizer runtime is loaded in a binary that doesn't really
support it, then __sanitizer_startup_hook will never have been
called to initialize StoredArgv. This case can't be supported, but
its failure mode shouldn't be to crash in sanitizer_common internals.

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D46344

llvm-svn: 331382