videotestsrc: Initialize buffer memory with zeroes

This prevents valgrind warnings when accessing the "x" parts
of xRGB and friends in other elements that handle (and can handle)
xRGB like ARGB (for example videoscale).

gst/videotestsrc/gstvideotestsrc.c

@@ -771,6 +771,8 @@ gst_video_test_src_create (GstPushSrc * psrc, GstBuffer ** buffer)
     gst_buffer_set_caps (outbuf, GST_PAD_CAPS (GST_BASE_SRC_PAD (psrc)));
   }
 
+  memset (GST_BUFFER_DATA (outbuf), 0, GST_BUFFER_SIZE (outbuf));
+
   if (src->pattern_type == GST_VIDEO_TEST_SRC_BLINK) {
     if (src->n_frames & 0x1) {
       gst_video_test_src_white (src, (void *) GST_BUFFER_DATA (outbuf),