baseaudiosrc: protect against ringbuffer disappearing while in a query

Observed a case where the src went to null-state during the query,
hence the spec pointer was no longer valid, and
gst_util_unit64_scale_int crashed (assertion `denom > 0´failed)

Add locking to make sure the ringbuffer can't disappear.

gst-libs/gst/audio/gstbaseaudiosrc.c

@@ -628,9 +628,12 @@ gst_base_audio_src_query (GstBaseSrc * bsrc, GstQuery * query)
       GstClockTime min_latency, max_latency;
       GstRingBufferSpec *spec;
 
+      GST_OBJECT_LOCK (src);
       if (G_UNLIKELY (src->ringbuffer == NULL
-              || src->ringbuffer->spec.rate == 0))
+              || src->ringbuffer->spec.rate == 0)) {
+        GST_OBJECT_UNLOCK (src);
         goto done;
+      }
 
       spec = &src->ringbuffer->spec;
 
@@ -642,6 +645,7 @@ gst_base_audio_src_query (GstBaseSrc * bsrc, GstQuery * query)
       max_latency =
           gst_util_uint64_scale_int (spec->segtotal * spec->segsize, GST_SECOND,
           spec->rate * spec->bytes_per_sample);
+      GST_OBJECT_UNLOCK (src);
 
       GST_DEBUG_OBJECT (src,
           "report latency min %" GST_TIME_FORMAT " max %" GST_TIME_FORMAT,