fix msan bug in pathops

Msan and Valgrind found an uninitialized memory mistake in
pathops. This also fixes similar bugs where not all parts
of the geometry were covered in the loop iteration.

R=borenet@google.com
NOTREECHECKS=true
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2366893003

Review-Url: https://codereview.chromium.org/2366893003
3 files changed