Fuzz

remove sketchy `LikelyFalse` error #314

Job	Run time
fuzz (roundtrip_semantic)	4m 48s
fuzz (parse_descriptor)	4m 45s
fuzz (parse_descriptor_secret)	5m 4s
fuzz (roundtrip_miniscript_script)	4m 55s
fuzz (roundtrip_miniscript_str)	4m 43s
fuzz (roundtrip_descriptor)	4m 44s
fuzz (roundtrip_concrete)	4m 40s
fuzz (compile_descriptor)	5m 11s
verify-execution	5s
	38m 55s

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