[Clang][Sema] Add nullptr check in IsFunctionConversion (#153710)

Static analysis flagged this code b/c ToFPT could be nullptr but we were
not checking it even though in the previous if statement we did. It
looks like this was a mistaken refactor from:

#135836

In the older code ToFPT was set using a cast which would have asserted
but no longer in the new code.

Author: shafik

clang/lib/Sema/SemaOverload.cpp

@@ -1979,7 +1979,7 @@ bool Sema::IsFunctionConversion(QualType FromType, QualType ToType,
   }
 
   // Drop 'noexcept' if not present in target type.
-  if (FromFPT) {
+  if (FromFPT && ToFPT) {
     if (FromFPT->isNothrow() && !ToFPT->isNothrow()) {
       FromFn = cast<FunctionType>(
           Context.getFunctionTypeWithExceptionSpec(QualType(FromFPT, 0),