Corrected crypto tag in SDP ANSWER for SRTP.

When Asterisk responds with an SDP ANSWER for SRTP, it had the code to
correctly fill in the crypto data, which was overwritten by a call to
sdp_crypto_offer. Corrected the situation by changing sdp_crypto_offer
to not replacing crypto data if it already exists.

(closes issue ASTERISK-20849)
Reported by: José Luis Millán
Tested by: Iñaki Baz Castillo
Patches:
	fix_sdp_crypto_tags.diff uploaded by Pedro Kiefer (license 6407)
........

Merged revisions 380043 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@380044 f38db490-d61c-443f-a65b-d21fe96a405b