libbpf: Fix potential multiplication overflow

Detected by LGTM static analyze in Github repo, fix potential multiplication
overflow before result is casted to size_t.

Fixes: 8505e870 ("libbpf: Implement generalized .BTF.ext func/line info adjustment")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200904041611.1695163-2-andriin@fb.com