xtensa: ISS: don't use string pointer before NULL check

Move strlen call inside the if block that checks string pointer for NULL.
While at it also fix the following coccicheck warning:

./arch/xtensa/platforms/iss/console.c:204:10-11: WARNING comparing
pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/platforms/iss/console.c

@@ -186,10 +186,10 @@ late_initcall(rs_init);
 
 static void iss_console_write(struct console *co, const char *s, unsigned count)
 {
-	int len = strlen(s);
-
-	if (s != 0 && *s != 0)
+	if (s && *s != 0) {
+		int len = strlen(s);
 		simc_write(1, s, count < len ? count : len);
+	}
 }
 
 static struct tty_driver* iss_console_device(struct console *c, int *index)