sk_*_push() returns the number of items on the stack, not the index of the

sk_*_push() returns the number of items on the stack, not the index of the
pushed item.  The index is the number of items - 1.  And if a NULL item was
found, actually use it.
Finally, provide a little bit of safety in CRYPTO_lock() by asserting the a
requested dynamic lock really must exist, instead of just being silent about it