Address static analyzer warning in OCPartialMockObject (Issue #543)

[LowAmmo]

• Issue OCPartialMockObject static analyzer warning #543 about potential nil passed to class_addMethod()

OCPartialMockObject#setupForwarderForSelector

• Modified to just return if there is no existing implementation for the method

[LowAmmo]

@erikdoe - Any chance this could be merged so it's in any future OCMock releases?

-Thanks!

[erikdoe]

To be honest, it's a long time since I wrote that code, but returning early from the method if originalIMP is null – as suggested in the PR – might be a bit too simple. I mean, there is the code to create types when originalMethod is null, and the forwarder is installed in such cases.

A less risky/invasive change would be to skip only the call to class_addMethod when originalIMP is null. This could have a performance impact, though, because this would skip setting any entry for aliasSelector` which means the if statement at the top of the method wouldn't be true on later invocations.

I think my assumption was that it was okay to set NULL as an implementation and that would trigger a forwarding mechanism. Looking at the header file for class_addMethod, that assumption is clearly wrong, at least today, and the third parameter must not be null. But that also means that it probably never is null. So, skipping the class_addMethod only to make the analyzer happy seems a sensible idea. (Famous last words...)