Handle the lack of a symbol table correctly.

Summary:
These two cases will trigger a dereference on a nullptr, since the
SymbolTable can be nonexistent for a given library, in addition to just
being empty.

Reviewers: alexshap

Reviewed By: alexshap

Subscribers: meikeb, kongyi, chh, jakehehrlich, llvm-commits, pirama

Differential Revision: https://reviews.llvm.org/D49534

llvm-svn: 338062