Add error return, upon mask creation failure, in sw_draw_with_mask_filter

Bug: 822680

Change-Id: I7296c5be4faf1f706ad1cc05198559771e324841
Reviewed-on: https://skia-review.googlesource.com/116180
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed