Change assert to graceful failure in SkRuntimeEffect

I'm not entirely sure what the difference is between our
path with the optimizer/inliner and that without is, but we
exercise it in our fuzzer [1] and the assert seems unnecessary.

[1] https://github.com/google/skia/blob/bdf369f26195b41c5d79212f85541ee3284c2080/fuzz/oss_fuzz/FuzzSkRuntimeBlender.cpp#L67

Bug: b/507643404
Change-Id: I9d0baf75a2f62a438f4aa6b3c5a2645186f15c3b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1225596
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>