perf_counter tools: Fix strbuf_fread() error path handling

size_t res cannot be less than 0 - fread returns 0 on error.

[ Updated by: René Scharfe <rene.scharfe@lsrfire.ath.cx> ]

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Junio C Hamano <gitster@pobox.com>
LKML-Reference: <4A3FB479.2090902@lsrfire.ath.cx>
Signed-off-by: Ingo Molnar <mingo@elte.hu>