Address static analyzer warning in OCPartialMockObject (Issue #543)

[LowAmmo]

• Issue #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!