Fail gracefully if MakeRenderTarget fails

We recently started passing --preAbandonGpuContext when running DM on
valgrind, resulting in this call returning nullptr. We had not been
checking for it, so we crash. Already fixed one in crrev.com/1959173002,
and now we reach another one.

BUG=skia:5282
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1972933002

Review-Url: https://codereview.chromium.org/1972933002