perf probe: Fix wrong warning in __show_one_line() if read(1) errors happen

This was introduced by commit fde52dbd.

Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <m3y67hsr0m.fsf@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>