[patch] MdePkg: check Length para before use in DevPathToTextUsbWWID

[Dandan Bi <dandan.bi@intel.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1336

In function DevPathToTextUsbWWID, the Length parameter is used
without check. This patch is to add check before using it.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
---
 MdePkg/Library/UefiDevicePathLib/DevicePathToText.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/MdePkg/Library/UefiDevicePathLib/DevicePathToText.c b/MdePkg/Library/UefiDevicePathLib/DevicePathToText.c
index 97d279eeb2..678f3d0a92 100644
--- a/MdePkg/Library/UefiDevicePathLib/DevicePathToText.c
+++ b/MdePkg/Library/UefiDevicePathLib/DevicePathToText.c
@@ -970,11 +970,11 @@ DevPathToTextUsbWWID (
 
   UsbWWId = DevPath;
 
   SerialNumberStr = (CHAR16 *) ((UINT8 *) UsbWWId + sizeof (USB_WWID_DEVICE_PATH));
   Length = (UINT16) ((DevicePathNodeLength ((EFI_DEVICE_PATH_PROTOCOL *) UsbWWId) - sizeof (USB_WWID_DEVICE_PATH)) / sizeof (CHAR16));
-  if (SerialNumberStr [Length - 1] != 0) {
+  if (Length >= 1 && SerialNumberStr [Length - 1] != 0) {
     //
     // In case no NULL terminator in SerialNumber, create a new one with NULL terminator
     //
     NewStr = AllocateCopyPool ((Length + 1) * sizeof (CHAR16), SerialNumberStr);
     ASSERT (NewStr != NULL);
-- 
2.18.0.windows.1