CloneV fix

[peteschultz]

This pull request address more fully the issue addressed in https://github.com/PetaVision/OpenPV/pull/281. In the AllocateDataStructures stage, CloneInternalStateBuffer was not properly checking that the original layer had allocated before setting its data pointer to the original layer's data pointer.

This pull request does the following:

• has CloneInternalStateBuffer override the allocateDataStructures() method to check whether the original InternalStateBuffer has allocated, and return POSTPONE if it hasn't.
• replaces the "if (V != nullptr)" in Austin's patch with a pvAssert(V != nullptr) statement, since the V pointer should never be null at this point in the code.

[garkenyon]

verified that this fixed seg fault during initialization (initialize state)