MdeModulePkg/UsbBusDxe: Add missing "return NULL" in UsbCreateDesc()

When (Len < Offset) is TRUE, indicating the data to visit is beyond
the boundary, the error message is printed but the function doesn't
return NULL.

It's a typo when modifying the commit 4c034bf62.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

MdeModulePkg/Bus/Usb/UsbBusDxe/UsbDesc.c

@@ -229,6 +229,7 @@ UsbCreateDesc (
   //
   if (Len < Offset) {
     DEBUG ((DEBUG_ERROR, "UsbCreateDesc: met mal-format descriptor, Offset/Len = %d/%d!\n", Offset, Len));
+    return NULL;
   }
 
   if ((Head->Type != Type) || (Head->Len < DescLen)) {