perf evsel: Don't rely on malloc working for sz 0

When running perf on ARC (uClibc based userspace), ran into this issue
   ------------->8----------------
	[ARCLinux]$ ./perf record ls
	bin             etc             perf            sys
	debug           init            perf.data       tmp
	[ perf record: Woken up 1 times to write data ]
	[ perf record: Captured and wrote 0.001 MB perf.data (~24 samples) ]

	[ARCLinux]$ ./perf report
	incompatible file format (rerun with -v to learn more)
   ------------->8----------------

The problem happens in the following call stack when zalloc is called
with size zero

glibc default / uClibc with MALLOC_GLIBC_COMPAT are OK, but not if that
config option is not enabled.

  cmd_report
     perf_session__new
	perf_session__open
	    perf_session__read_header
		read_attr(fd, header, &f_attr)
		nr_ids = f_attr.ids.size / sizeof(u64); <-- 0
		perf_evsel__alloc_id(vsel, 1, nr_ids)
			zalloc(ncpus * nthreads * sizeof(u64)) <-- 0

header.c: read_attr()

(gdb) p *f_attr
$17 = {
  attr = {
    type = 0,
    size = 96,
    config = 0,
    {
      sample_period = 4000,
      sample_freq = 4000
    },
...
  ids = {
    offset = 104,
    size = 0      <------
  }
}

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1421156604-30603-5-git-send-email-vgupta@synopsys.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.c

@@ -797,6 +797,9 @@ int perf_evsel__enable(struct perf_evsel *evsel, int ncpus, int nthreads)
 
 int perf_evsel__alloc_id(struct perf_evsel *evsel, int ncpus, int nthreads)
 {
+	if (ncpus == 0 || nthreads == 0)
+		return 0;
+
 	if (evsel->system_wide)
 		nthreads = 1;