videoaggregator: Fix string leak

gst_video_colorimetry_to_string() returns allocated memory which
must be freed.

https://bugzilla.gnome.org/show_bug.cgi?id=796596
diff --git a/gst-libs/gst/video/gstvideoaggregator.c b/gst-libs/gst/video/gstvideoaggregator.c
index 383beb3..b155ff0 100644
--- a/gst-libs/gst/video/gstvideoaggregator.c
+++ b/gst-libs/gst/video/gstvideoaggregator.c
@@ -661,13 +661,14 @@
     gst_caps_unref (tmp);
   }
 
+  color_name = gst_video_colorimetry_to_string (&best_info.colorimetry);
+
   GST_DEBUG_OBJECT (vagg,
       "The output format will now be : %d with chroma : %s and colorimetry %s",
       best_format, gst_video_chroma_to_string (best_info.chroma_site),
-      gst_video_colorimetry_to_string (&best_info.colorimetry));
+      color_name);
 
   best_format_caps = gst_caps_copy (caps);
-  color_name = gst_video_colorimetry_to_string (&best_info.colorimetry);
   gst_caps_set_simple (best_format_caps, "format", G_TYPE_STRING,
       gst_video_format_to_string (best_format), "chroma-site", G_TYPE_STRING,
       gst_video_chroma_to_string (best_info.chroma_site), "colorimetry",