libbpf: Deal with section with no data gracefully

libbpf: Deal with section with no data gracefully

[ Upstream commit 35a855509e6ee3442477c8ebc6827b5b5d32a7b5 ]

ELF section data pointer returned by libelf may be NULL (if section has
SHT_NOBITS), so null check section data pointer before attempting to
copy license and kversion section.

Fixes: cb1e5e961991 ("bpf tools: Collect version and license from ELF sections")
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221012022353.7350-3-shung-hsi.yu@suse.com
Signed-off-by: Sasha Levin <sashal@kernel.org>