libass: avoid negative index in array

(cherry picked from commit e91ea1f9c3f5575c65003818b6b0c2948435c2b8)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/libass.c

@@ -627,6 +627,8 @@ static int BuildRegions( rectangle_t *p_region, int i_max_region, ASS_Image *p_i
 #ifdef DEBUG_REGION
             msg_Err( p_spu, "Merging %d and %d", i_best_i, i_best_j );
 #endif
+            if( i_best_j >= 0 && i_best_i >= 0 )
+            {
                 r_add( &region[i_best_i], &region[i_best_j] );
 
                 if( i_best_j+1 < i_region )
@@ -634,6 +636,7 @@ static int BuildRegions( rectangle_t *p_region, int i_max_region, ASS_Image *p_i
                 i_region--;
             }
         }
+    }
 
     /* */
     for( int n = 0; n < i_region; n++ )