Issue #1974: Memory leak in gimpimage.c.

Ok my previous fix was wrong (at least for the part in the macro). This
is a macro, not a function. So each time we write _reason, the call to
g_strdup_printf() is reevaluated, hence data is allocated.
The right fix is to prepend `tmp` to the list, not `_reason`.
Thanks to Massimo for the debugging, as always!

app/core/gimpimage.c

@@ -2373,8 +2373,7 @@ gimp_image_get_xcf_version (GimpImage    *image,
     if (g_list_find_custom (reasons, tmp, (GCompareFunc) strcmp)) \
       g_free (tmp);                                               \
     else                                                          \
-      reasons = g_list_prepend (reasons, (_reason)); }            \
-  else g_free (_reason);
+      reasons = g_list_prepend (reasons, tmp); }
 
   /* need version 1 for colormaps */
   if (gimp_image_get_colormap (image))