perf pmu: Add proper error handling to print_pmu_events()

perf pmu: Add proper error handling to print_pmu_events()

It was silently returning or printing "(null)" when no memory was
available at various points. Fix it by checking and warning the user
when that happens.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-835udmf66x9nza504cu6irz9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>