vbo: initialize num_instances in a few places

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=33247
There might still be some issues with drawing multiple instances
with VBO splitting to investigate someday.

src/mesa/vbo/vbo_split_copy.c

@@ -222,6 +222,7 @@ begin( struct copy_context *copy, GLenum mode, GLboolean begin_flag )
 
    prim->mode = mode;
    prim->begin = begin_flag;
+   prim->num_instances = 1;
 }
 
 

src/mesa/vbo/vbo_split_inplace.c

@@ -222,6 +222,7 @@ static void split_prims( struct split_context *split)
 	 tmpprim.indexed = 1;
 	 tmpprim.start = 0;
 	 tmpprim.count = count;
+         tmpprim.num_instances = 1;
 
 	 flush_vertex(split);