va_list 'ap' was opened but not closed by va_end() in error case. It should
be closed by va_end() before the return.

Fixes: aa52bcbe ("tools: bpftool: Fix json dump crash on powerpc")
Signed-off-by: Gu Shengxian <gushengxian@yulong.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/bpf/20210706013543.671114-1-gushengxian507419@gmail.com