perf utils: use sizeof(buf) - 1 in readlink() call

Ensure that we have space for the null byte in buf.
Signed-off-by: NTommi Rantala <tommi.t.rantala@nokia.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170322130624.21881-5-tommi.t.rantala@nokia.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -378,7 +378,7 @@ static int write_cmdline(int fd, struct perf_header *h __maybe_unused,
 	 * actual atual path to perf binary
 	 */
 	sprintf(proc, "/proc/%d/exe", getpid());
-	ret = readlink(proc, buf, sizeof(buf));
+	ret = readlink(proc, buf, sizeof(buf) - 1);
 	if (ret <= 0)
 		return -1;