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