perf: Fix multi-event parsing bug

This patch fixes an issue with event parsing.
The following commit appears to have broken the
ability to specify a comma separated list of events:

   commit ceb53fbf
   Author: Ingo Molnar <mingo@elte.hu>
   Date:   Wed Apr 27 04:06:33 2011 +0200

       perf stat: Fail more clearly when an invalid modifier is specified

This patch fixes this while preserving the desired effect:

$ perf stat -e instructions:u,instructions:k ls /dev/null /dev/null

 Performance counter stats for 'ls /dev/null':

            365956 instructions:u           #    0.00  insns per cycle
            731806 instructions:k           #    0.00  insns per cycle

        0.001108862  seconds time elapsed

$ perf stat -e task-clock-msecs true
invalid event modifier: '-msecs'
Run 'perf list' for a list of valid events and modifiers
Signed-off-by: NStephane Eranian <eranian@google.com>
Cc: acme@redhat.com
Cc: peterz@infradead.org
Cc: fweisbec@gmail.com
Link: http://lkml.kernel.org/r/20110517133619.GA6999@quadSigned-off-by: NIngo Molnar <mingo@elte.hu>

tools/perf/util/parse-events.c

@@ -734,6 +734,9 @@ parse_event_modifier(const char **strp, struct perf_event_attr *attr)
 	if (!*str)
 		return 0;
 
+	if (*str == ',')
+		return 0;
+
 	if (*str++ != ':')
 		return -1;