Issue #1731: gbr file that crashes gimp-2.10.

GIMP_BRUSH_MAX_SIZE was already defined (as 10.000 pixels per dimension,
which is big for a brush) in gimpbrush.h. Let's just use this to
validate the size returned by the header.

(cherry picked from commit b3de0bb7a5)

app/core/gimpbrush-load.c

@@ -34,7 +34,6 @@
 
 #include "gimp-intl.h"
 
-
 /* stuff from abr2gbr Copyright (C) 2001 Marco Lamberto <lm@sunnyspot.org>  */
 /* the above is GPL  see http://the.sunnyspot.org/gimp/  */
 
@@ -189,6 +188,15 @@ gimp_brush_load_brush (GimpContext   *context,
       return NULL;
     }
 
+  if (header.width > GIMP_BRUSH_MAX_SIZE ||
+      header.height > GIMP_BRUSH_MAX_SIZE)
+    {
+      g_set_error (error, GIMP_DATA_ERROR, GIMP_DATA_ERROR_READ,
+                   _("Fatal parse error in brush file: %dx%d over max size."),
+                   header.width, header.height);
+      return NULL;
+    }
+
   switch (header.version)
     {
     case 1: