perf stat: fix NULL pointer reference bug with event unit

This patch fixes a problem with the handling of the newly introduced
optional event unit. The following cmdline caused a segfault:

 $ perf stat -e cpu/event-0x3c/ ls

This patch fixes the problem with the default setting for alias->unit
which was eventually causing the segfault.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1389972846-6566-2-git-send-email-eranian@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
index a7f1b6a..d248fca 100644
--- a/tools/perf/util/parse-events.c
+++ b/tools/perf/util/parse-events.c
@@ -635,7 +635,7 @@
 	struct perf_event_attr attr;
 	struct perf_pmu *pmu;
 	struct perf_evsel *evsel;
-	char *unit;
+	const char *unit;
 	double scale;
 
 	pmu = perf_pmu__find(name);