plug-ins: fix memory leak in file-mng

Found using clang's scan-build.

plug-ins/common/file-mng.c

@@ -469,7 +469,9 @@ respin_cmap (png_structp  pp,
   /* Make sure there is something in the colormap */
   if (colors == 0)
     {
-      before = g_new0 (guchar, 3);
+      before = g_newa (guchar, 3);
+      memset (before, 0, sizeof (guchar) * 3);
+
       colors = 1;
     }