res_pjsip_registrar.c: Prevent possible buffer overflow with domain aliases

res_pjsip_registrar.c: Prevent possible buffer overflow with domain aliases

We're appropriately sizing the id_domain_alias buffer, but then copying the data
into the id_domain one. We were then using the uninitialized id_domain_alias
buffer we just allocated.

This is ASTERISK~28641 adjacent, but significant enough to warrant its own
patch.

Change-Id: I81c38724d18deab8c6573153e2b99dbb6e2f33d9