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: 8646db23 ("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