Fix a harmless assertion fault discovered by

[https://oss-fuzz.com/testcase-detail/5517690440646656|OSSFuzz].  The
assertion fault is harmless because the assert() checks a condition that
that is harmless if false and the assert() is disabled in production
builds.

FossilOrigin-Name: 79443aabe1db8cc2ff9537fde0dac0e858eae3f3848ee9cc1b8fba47b824bf08