fix path ops fuzz bug

fix path ops fuzz bug

If one path is empty and the other has extreme values, the
intermediate coincident paths cannot be resolved, but triggers
an assert that a data structure unexpectedly has zero-length.

Tunnel this failure back up to the top and return that the
entire path op fails.

A future optimization could detect the empty path and avoid
this, allowing the op to succeed -- not sure that it's worth
the additional logic though.

TBR=reed@google.com
BUG=535151
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1730293002

Review URL: https://codereview.chromium.org/1730293002