res_pjsip_sdp_rtp.c: Don't alter global addr variable.

* create_rtp(): Fix unexpected alteration of global address_rtp if a
transport is bound to an address.

* create_rtp(): Fix use of uninitialized memory if the endpoint RTP media
address is invalid or the transport has an invalid address.

ASTERISK-26851

Change-Id: Icde42e65164a88913cb5c2601b285eebcff397b7