perf top: Fix uninitialized 'counter' variable

builtin-top.c has an uninitialized variable.
gcc(version 4.5.1) warns about it and it results in build failure:

 builtin-top.c: In function 'display_thread':
 builtin-top.c:518:9: error: 'counter' may be used uninitialized

This situation can indeed trigger, if the getline() call in
prompt_integer() fails.
Signed-off-by: Akihiro Nagai <akihiro.nagai.hw@hitachi.com>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110323072939.11638.50173.stgit@localhost6.localdomain6>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

tools/perf/builtin-top.c

@@ -515,7 +515,9 @@ static void handle_keypress(struct perf_session *session, int c)
 			break;
 		case 'E':
 			if (top.evlist->nr_entries > 1) {
-				int counter;
+				/* Select 0 as the default event: */
+				int counter = 0;
+
 				fprintf(stderr, "\nAvailable events:");
 
 				list_for_each_entry(top.sym_evsel, &top.evlist->entries, node)