res_pjsip_authenticator_digest: Fix md5 hash buffer

An md5 hash is 32 bytes long. The char buffer must be at least 33 bytes to
avoid clobbering of the stack. This patch also fixes a potential clobbering
in test_utils.c.

Thanks to Andrew Nagy for reporting and testing this out in #asterisk-dev

Reported by: Andrew Nagy
Tested by: Andrew Nagy
........

Merged revisions 404843 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404844 65c4cc65-6c06-0410-ace0-fbb531ad65f3