perf config: Check error cases of {show_spec, set}_config()

perf config: Check error cases of {show_spec, set}_config()

show_spec_config() and set_config() can be called multiple times
in the loop in cmd_config().

However, The error cases of them wasn't checked, so fix it.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1497671197-20450-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>