Backport checkin: Fix a segfault when b"" was passed to b2a_qp() -- it was using strchr() instead of memchr().
Backport checkin:
Fix a segfault when b"" was passed to b2a_qp() -- it was using strchr()
instead of memchr().

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator