chan_sip: Clear ToHost property on peer when changing to dynamic host

The ToHost parameter was not cleared when a peer's host value was
changed to dynamic. This causes invites to be sent to the original host.

ASTERISK-29011 #close

Change-Id: I9678d512741f71baca8f131a65b7523020b07d5c

channels/chan_sip.c

@@ -31903,6 +31903,7 @@ static struct sip_peer *build_peer(const char *name, struct ast_variable *v_head
 					if ((!found && !ast_test_flag(&global_flags[1], SIP_PAGE2_RTCACHEFRIENDS)) || !peer->host_dynamic) {
 						/* Initialize stuff if this is a new peer, or if it used to
 						 * not be dynamic before the reload. */
+						ast_string_field_set(peer, tohost, NULL);
 						ast_sockaddr_setnull(&peer->addr);
 					}
 					peer->host_dynamic = TRUE;