perf tools: Fix inverted error verification bug in thread__fork

Commit 1902efe7 for the new comm infra added the wrong check for return
code on thread__set_comm. err == 0 is normal, so don't return at that
point unless err != 0.

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1386736538-23525-1-git-send-email-dsahern@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>