libbpf: Fix memory leak in btf__dedup()

Free btf_dedup if btf_ensure_modifiable() returns error.

Fixes: 919d2b1d ("libbpf: Allow modification of BTF and add btf__add_str API")
Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211022202035.48868-1-mauricio@kinvolk.io