intel: add null src pointer check in intel_region_reference()

Fixes segfault when running cubemap demo on i945.  This happened
when intel_region_reference() was called in i915_set_draw_region()
with depth_region=NULL.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/intel/intel_regions.c

@@ -271,7 +271,8 @@ intel_region_reference(struct intel_region **dst, struct intel_region *src)
       if (*dst)
 	 intel_region_release(dst);
 
-      src->refcount++;
+      if (src)
+         src->refcount++;
       *dst = src;
    }
 }