Fix two issues:

1. The default with of hex value is 0 according to MdeLib spec. If we expect it to be 16, we need to state it in the format string explicitly.
2. For UnicodeSPrint, the output buffer size of 0 will not print anything to the buffer according to MdeLib spec. We need to specify the maximum possible buffer size.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4635 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/DevicePathDxe/DevicePathToText.c

@@ -440,11 +440,11 @@ EisaIdToText (
   //
   //UnicodeSPrint ("%X", 0x0a03) => "0000000000000A03"
   //
-  UnicodeSPrint (PnpIdStr, 17 * 2, L"%X", EisaId >> 16);
+  UnicodeSPrint (PnpIdStr, 17 * 2, L"%16X", EisaId >> 16);
 
   UnicodeSPrint (
     Text,
-    0,
+    sizeof (CHAR16) + sizeof (CHAR16) + sizeof (CHAR16) + sizeof (PnpIdStr),
     L"%c%c%c%s",
     '@' + ((EisaId >> 10) & 0x1f),
     '@' + ((EisaId >>  5) & 0x1f),