audiovisualizer: shader: Fix dframe out of bound write

shader_fade_and_move_horiz_out writes a complete stride at the end of its
dframe data

This led to SIGSEGV since the stride reached into sframe->map[0]->memory
which could not be umpapped later on.

This is due to `d` is increased twice. Once at the end of the upper loop
and at the start of the lower loop.
The corresponding dframe stride is therefore skipped.

Rewind `d` and start at the correct position.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/1702

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1765>

subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstaudiovisualizer.c

@@ -336,6 +336,10 @@ shader_fade_and_move_horiz_out (GstAudioVisualizer * scope,
     }
     d += ds;
   }
+
+  /* rewind one stride */
+  d -= ds;
+
   /* move lower half down */
   for (j = 0; j < height / 2; j++) {
     d += ds;