src: add refcount asserts

src: add refcount asserts

_get() functions must not be used when refcnt is 0, as expr_free()
releases expressions on 1 -> 0 transition.

Also, check that a refcount would not overflow from UINT_MAX to 0.
Use INT_MAX to also catch refcount leaks sooner, we don't expect
2**31 get()s on same object.

This helps catching use-after-free refcounting bugs even when nft
is built without ASAN support.

v3: use a macro + BUG to get more info without a coredump.

Signed-off-by: Florian Westphal <fw@strlen.de>