check for zero conic weights in interp

this will fix the current (and uncover the next)
ASAN failure

TBR=tomhudson@google.com
BUG=skia:4549
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1711813002

Review URL: https://codereview.chromium.org/1711813002
1 file changed