Fuzz crashes

[kiminuo]

Recent build failures https://travis-ci.com/github/apoelstra/rust-miniscript/builds seems to be linked with fuzzing crashes.

So far my understanding of the problem is that there occurrences of &top.args[0] in astelem.rs file (e.g. https://github.com/apoelstra/rust-miniscript/blob/96105bb45c6efc60d0bac41866e4786b2276ed67/src/miniscript/astelem.rs#L448) and it is not being checked that top.args really does have that many arguments.

No guarantees at this point. though.