UBSan crash with zero-length input

We are suddenly getting unreproducible UBSan crashes with zero-length input, see https://oss-fuzz.com/testcase-detail/4680147175276544 The crash seems to happen not in our code, but specifically in UBSan.

Unreproducible crashes on zero-length input were a long-standing problem on i386 (see #7421) but I never saw them on x86_64.

Start with OSS-Fuzz testcase 4680147175276544 and compare the reported zero-length-input behavior with issue #7421. Determine whether the crash is in UBSan or the fuzzed project, and use a reproducible case or documented findings to establish when the issue is resolved.

Written by the indexing model from the issue text.