perf pmu: Make pmu_formats_string to check return value of strbuf

Make pmu_formats_string() to check return value of strbuf APIs so that
it can detect errors in it.

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/20160510054744.6158.37810.stgit@devbox


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