Protect against potential null pointer dereference

[pzmarzly]

In this code, set can be NULL, in which case accessing set->key is a null pointer dereference.

This case is currently unreachable - _bf_matcher_generate_set_trie is only called by bf_chain_get_set_for_matcher, which validates the set exists first.

However, for defensive programming, we should check for if (!set) here as well, and error out if it's missing.

bpfilter/src/libbpfilter/cgen/matcher/set.c

Lines 15 to 23 in e289171

	static int _bf_matcher_generate_set_trie(struct bf_program *program,
	const struct bf_matcher *matcher)
	{
	assert(program);
	assert(matcher);
	const struct bf_set *set =
	bf_chain_get_set_for_matcher(program->runtime.chain, matcher);
	enum bf_matcher_type type = set->key[0];

This issue was found by cppcheck and validated by Claude Code.

[yaakov-stein]

I noticed this one while I was refactoring the set.h/c files for #493. Fix should already be included in there, so we can close this once it gets merged.