wrap verifier calls in catch_unwind

[larskuhtz]

This prevents the Rust FFI calls from panicking and terminating the program with SIGABRT. Instead the verifier indicates an unsuccessful validation to the caller.

It is still the obligation of the caller to insure that the arguments are valid non-null null-terminated C-strings.

I would consider it a preliminary solution. Ideally, the verifier should return an error code (and/or possibly a message) on failure that that indicates what went wrong and allows the application to provide meaningful feedback to the user. (e.g. "invalid input encoding" vs. "proof validation failed").

[wwared]

I haven't tested this PR yet, but it's worth mentioning it might be worth looking at RFC 2945 and possibly using "C-unwind" abi instead of "C" if we are using catch_unwind.

[huitseeker]

This fixes the control flow issue for the node code.

We'll need to fix #2 for better error management, which should probably be addressed at the level of the Go FFI.