don't fix winding for empty paths

don't fix winding for empty paths

A path can be non-empty but become empty when it is simplified.
For instance, a path with the same rectangle, twice, with opposite
windings.

No contours are created for empty paths, so don't try to
fix their winding direction.

Additionally, check for a NULL coincidence since the
OpBuilder assumes that no concidence edges can be present
after the paths are simplified. This code should not get
called, but it's worth the future-proofing to check.

R=fmalita@chromium.org
BUG=502792
Committed: https://skia.googlesource.com/skia/+/218f21ac09c70b98a10cb8e1999b85a22fa0b151

[caryclark, 2015-06-29 18:34:38]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2015-06-29 18:34:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1218863005/1

[commit-bot: I haz the power, 2015-06-29 18:34:48]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-06-30 00:34 UTC

[f(malita), 2015-06-29 18:35:53]

lgtm

[commit-bot: I haz the power, 2015-06-29 18:41:58]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/218f21ac09c70b98a10cb8e1999b85a22fa0b151