Bluetooth: shell: Fix accessing invalid memory

argc needs to be check before accessing argv.

Change-Id: I9cb70906a388b96df4e192dd4f31eafdab25127f
Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>

tests/bluetooth/shell/src/main.c

@@ -1404,6 +1404,10 @@ static bool hrs_simulate;
 
 static int cmd_hrs_simulate(int argc, char *argv[])
 {
+	if (argc < 2) {
+		return -EINVAL;
+	}
+
 	if (!strcmp(argv[1], "on")) {
 		static bool hrs_registered;