soo_stat: Ensure error is always initialized.

In kernels without MAC, error is not set for sockets whose protocol
layer does not implement the pr_sense hook.

Reported by:	Jenkins (powerpc kernel builds)
Fixes:		7c04ca1fad67 sockets: for stat(2) on a socket don't report hiwat as block size

sys/kern/sys_socket.c

@@ -303,7 +303,7 @@ static int
 soo_stat(struct file *fp, struct stat *ub, struct ucred *active_cred)
 {
 	struct socket *so = fp->f_data;
-	int error;
+	int error = 0;
 
 	bzero((caddr_t)ub, sizeof (*ub));
 	ub->st_mode = S_IFSOCK;