[tracer] fix bogus multiple tracing

When calling the trace() function multiple times from script (to do tracing via different backends), be sure and install all the tracing callbacks (in TM, dialog) only first time.
Reported by @gmaruzz
Closes #3082

modules/tracer/tracer.c

@@ -1930,7 +1930,13 @@ static int sip_trace_handle(struct sip_msg *msg, tlist_elem_p el,
 		info->instances = instance;
 	}
 
-	if (trace_flags==TRACE_B2B) {
+	if (info->instances->next) {
+		/* this is not the first instance to be added, which means
+		 * we need only to add the instance to the list (already done above).
+		 * All the tracing callbacks are already present (set when the first
+		 * tracing instance was created), so nothing to do from this
+		 * perspective */
+	} else if (trace_flags==TRACE_B2B) {
 		if (trace_b2b(msg, info) < 0) {
 			LM_ERR("trace b2b failed!\n");
 			return -1;