staging: r8188eu: get a string from the user correctly

The original code had two bugs:
1) It didn't check if the string was zero length so it could oops when
   it tried to dereference the ZERO_SIZE_PTR.
2) It didn't enforce that the string was NUL terminated.

It was also messy as pants.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/os_dep/rtw_android.c

@@ -162,22 +162,12 @@ int rtw_android_priv_cmd(struct net_device *net, struct ifreq *ifr, int cmd)
 		ret = -EFAULT;
 		goto exit;
 	}
-	command = kmalloc(priv_cmd.total_len, GFP_KERNEL);
-	if (!command) {
-		DBG_88E("%s: failed to allocate memory\n", __func__);
-		ret = -ENOMEM;
-		goto exit;
-	}
-	if (!access_ok(VERIFY_READ, priv_cmd.buf, priv_cmd.total_len)) {
-		DBG_88E("%s: failed to access memory\n", __func__);
-		ret = -EFAULT;
-		goto exit;
-	}
-	if (copy_from_user(command, (char __user *)priv_cmd.buf,
-			   priv_cmd.total_len)) {
-		ret = -EFAULT;
-		goto exit;
-	}
+	if (priv_cmd.total_len < 1)
+		return -EINVAL;
+	command = memdup_user(priv_cmd.buf, priv_cmd.total_len);
+	if (IS_ERR(command))
+		return PTR_ERR(command);
+	command[priv_cmd.total_len - 1] = 0;
 	DBG_88E("%s: Android private cmd \"%s\" on %s\n",
 		__func__, command, ifr->ifr_name);
 	cmd_num = rtw_android_cmdstr_to_num(command);