bpf: Fix a crash when btf_parse_base() returns an error pointer

The pointer returned by btf_parse_base could be an error pointer.
IS_ERR() check is needed before calling btf_free(base_btf).

Fixes: 8646db2 ("libbpf,bpf: Share BTF relocate-related code with kernel")
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240830012214.1646005-1-martin.lau@linux.dev

Author: Martin KaFai Lau

kernel/bpf/btf.c

@@ -6283,7 +6283,7 @@ static struct btf *btf_parse_module(const char *module_name, const void *data,
 
 errout:
 	btf_verifier_env_free(env);
-	if (base_btf != vmlinux_btf)
+	if (!IS_ERR(base_btf) && base_btf != vmlinux_btf)
 		btf_free(base_btf);
 	if (btf) {
 		kvfree(btf->data);