Merge branch 'bpf-bpftool-json-fixes'

Quentin Monnet says:

====================
These are two minor fixes to avoid breaking JSON output in batch mode. The
first one makes bpftool output a "null" JSON object, as expected in batch
mode if nothing else is to be printed, when dumping program instructions
into an output file. The second one replaces a call to "perror()" with
something that does not break JSON when parsing input file for batch mode.
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>