This changes all the extern C functions in `hyper::ffi` to check passed
pointer arguments for being `NULL` before trying to use them. Before, we
would just assume the programmer had passed a good pointer, which could
result in segmentation faults. Now:
- In debug builds, it will assert they aren't null, and so if they are,
a message identifying the argument name will be printed and then the
process will crash.
- In release builds, it will still check for null, but if found, it will
return early, with a return value indicating failure if the return type
allows (such as returning NULL, or `HYPERE_INVALID_ARG`).
Closes#2620
Make C API functions that return pointers return null in case of a
panic, instead of aborting.
Add ffi_fn! macro rules that enable default error values to be returned
by writing "?= <value>" after an ffi function's body.
