tools: bpftool: return from do_event_pipe() on bad arguments

When command line parsing fails in the while loop in do_event_pipe()
because the number of arguments is incorrect or because the keyword is
unknown, an error message is displayed, but bpftool remains stuck in
the loop. Make sure we exit the loop upon failure.

Fixes: f412eed9

 ("tools: bpftool: add simple perf event output reader")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>