FS-3882 please try this patch

src/mod/endpoints/mod_rtmp/rtmp.c

@@ -886,6 +886,11 @@ switch_status_t rtmp_handle_data(rtmp_session_t *rsession)
 							if (rsession->tech_pvt) {
 								uint16_t len = state->origlen;
 								
+								if (!rsession->tech_pvt->readbuf) {
+									return SWITCH_STATUS_FALSE;
+								}
+
+								
 								switch_mutex_lock(rsession->tech_pvt->readbuf_mutex);
 								if (rsession->tech_pvt->maxlen && switch_buffer_inuse(rsession->tech_pvt->readbuf) > rsession->tech_pvt->maxlen * 40) {
 									rsession->tech_pvt->over_size++;