Fuzz

remove sketchy `LikelyFalse` error #313

Job	Run time
fuzz (roundtrip_semantic)	4m 58s
fuzz (parse_descriptor)	4m 40s
fuzz (parse_descriptor_secret)	4m 41s
fuzz (roundtrip_miniscript_script)	4m 51s
fuzz (roundtrip_miniscript_str)	4m 58s
fuzz (roundtrip_descriptor)	4m 57s
fuzz (roundtrip_concrete)	4m 48s
fuzz (compile_descriptor)	4m 40s
verify-execution	4s
	38m 37s

Provide feedback