Add defensive check that argument may be null in call to compareConversionFunctions() on incorrect code.

I'm looking into getting a reduced test case, but it's not
immediately available.

Fixes <rdar://problem/16344806>

llvm-svn: 205285

clang/lib/Sema/SemaOverload.cpp

@@ -3278,7 +3278,7 @@ compareConversionFunctions(Sema &S, FunctionDecl *Function1,
   //   respectively, always prefer the conversion to a function pointer,
   //   because the function pointer is more lightweight and is more likely
   //   to keep code working.
-  CXXConversionDecl *Conv1 = dyn_cast<CXXConversionDecl>(Function1);
+  CXXConversionDecl *Conv1 = dyn_cast_or_null<CXXConversionDecl>(Function1);
   if (!Conv1)
     return ImplicitConversionSequence::Indistinguishable;