mod_sofia: Follow-up fix for FS-5009 to avoid NULL-ptr dereference

Commit f913670e changed switch_core_session_sprintf() args
from sip->sip_to to sip->sip_from, but didn't update the enclosing if()
clause protecting the code from dereferencing NULL pointers.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

src/mod/endpoints/mod_sofia/sofia.c

@@ -7753,7 +7753,7 @@ void sofia_handle_sip_i_invite(switch_core_session_t *session, nua_t *nua, sofia
 			calling_myself++;
 		}
 
-		if (sip && sip->sip_to) {
+		if (sip && sip->sip_from) {
 			user = switch_core_session_sprintf(session, "%s@%s", sip->sip_from->a_url->url_user, sip->sip_from->a_url->url_host);
 			switch_ivr_set_user(session, user);
 		}