[libclang] Fix error handler in translateSourceLocation.

Given an invalid SourceLocation, translateSourceLocation will call
clang_getNullLocation, and then do nothing with the result. But
clang_getNullLocation has no side effects: it just constructs and
returns a null CXSourceLocation value.

Surely the intention was to //return// that null CXSourceLocation to
the caller, instead of throwing it away and pressing on anyway.

Reviewed By: miyuki

Differential Revision: https://reviews.llvm.org/D104442