llvm-c: Return NULL from LLVMGetFirstTarget instead of asserting

If no targets are registered, LLVMGetFirstTarget currently fails with
an assertion. This patch makes it return NULL instead, similarly to
how LLVMGetNextTarget would.

Patch by Peter Zotov

Differential Revision: http://llvm-reviews.chandlerc.com/D1908

llvm-svn: 192878