libass: avoid crash on ASS_Images which have width or height == 0

modules/codec/libass.c

@@ -449,6 +449,7 @@ static int BuildRegions( spu_t *p_spu, rectangle_t *p_region, int i_max_region,
 #endif
 
     for( p_tmp = p_img_list, i_count = 0; p_tmp != NULL; p_tmp = p_tmp->next )
+        if( p_tmp->w > 0 && p_tmp->h > 0 )
             i_count++;
     if( i_count <= 0 )
         return 0;
@@ -457,8 +458,9 @@ static int BuildRegions( spu_t *p_spu, rectangle_t *p_region, int i_max_region,
     if( !pp_img )
         return 0;
 
-    for( p_tmp = p_img_list, i_count = 0; p_tmp != NULL; p_tmp = p_tmp->next, i_count++ )
-        pp_img[i_count] = p_tmp;
+    for( p_tmp = p_img_list, i_count = 0; p_tmp != NULL; p_tmp = p_tmp->next )
+        if( p_tmp->w > 0 && p_tmp->h > 0 )
+            pp_img[i_count++] = p_tmp;
 
     /* */
     const int i_w_inc = __MAX( ( i_width + 49 ) / 50, 32 );