SF bug #494738: binascii_b2a_base64 overwrites memory.

binascii_b2a_base64():  We didn't allocate enough buffer space for very
short inputs (e.g., a 1-byte input can produce a 5-byte output, but we
only allocated 2 bytes).  I expect that malloc overheads absorbed the
overrun in practice, but computing a correct upper bound is a very simple
change.