perf help: Make check_emacsclient_version to check strbuf APIs

Make check_emacsclient_version() to check the return value of strbuf
APIs so that it can handle errors in strbuf.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20160510054716.6158.11755.stgit@devbox


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