Don't crash if pa_stream_peek returns NULL

pa_stream_peek can return NULL if either the buffer is empty or if it
has a hole. In either case we need to avoid derefencing the data
pointer. Additionally, if there is a hole, we need to call pa_stream_drop,
if the buffer is empty we should not call it.

BugLink: https://bugs.debian.org/735898

src/mainwindow.cc

@@ -493,6 +493,14 @@ static void read_callback(pa_stream *s, size_t length, void *userdata) {
         return;
     }
 
+    if (!data) {
+        /* NULL data means either a hole or empty buffer.
+         * Only drop the stream when there is a hole (length > 0) */
+        if (length)
+            pa_stream_drop(s);
+        return;
+    }
+
     assert(length > 0);
     assert(length % sizeof(float) == 0);