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Avoid potential null-deref in active_edges()

This fixes a crash found by a fuzzer-generated testcase in Firefox where the right pointer is null; see https://bugzilla.mozilla.org/show_bug.cgi?id=1797103. Unfortunately I don't have a simple/standalone test that reaches this, and am not really sure if this is the right way to fix, or if something went wrong earlier leaving the polygons in an unexpected state, but this at least resolves the observed crash.

It shouldn't change behavior for any currently-working case, because it only affects the case where we're otherwise about to crash.

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