In order to fix SF bug # 824977, we replace calloc()/free() calls in

binascii_a2b_qp() and binascii_b2a_qp() with calls to PyMem_Malloc() and
PyMem_Free().  These won't return NULL unless the allocations actually fail,
so it won't trigger a bogus memory error on some platforms <cough>AIX</cough>
when passed a length of zero.