Misc changes for .18

* app/indexed_palette.c: fix for wrong color selected in indexed
        palette dialog

        * app/xcf.c: don't crash on bad input (0 byte files)

        * app/plug_in.h
        * app/plug_in.c: fixes Gimp's most obscure bug. Failed plugin
        queries are handle correctly now

        * app/commands.c: added marching ants speed to preferences

        * plug-ins/tiff/tiff.c: correction for inversion for MINISWHITE
        images without alpha

        * plug-ins/pcx/pcx.c: updated to new version

        * app/paint_funcs.h: changed OPAQUE and TRANSPARENT to
        OPAQUE_OPACITY and TRANSPARENT_OPACITY to avoid possible
        conflicts. All affects .c files changed.

-Yosh

app/gui/plug-in-commands.c

@@ -1967,9 +1967,8 @@ plug_in_query (char      *filename,
 	    }
 
 	  plug_in_pop ();
+	  plug_in_destroy (plug_in);
 	}
-
-      plug_in_destroy (plug_in);
     }
 }