Fix debug message output issue

The latest code has debug message output issue. It is caused by
the following CommitId: 56867c3bc6.
This patch provided proper fixes for this issue. The root cause
is due to incorrect string length.

Signed-off-by: Maurice Ma <maurice.ma@intel.com>

BootloaderCommonPkg/Library/BootloaderDebugLib/DebugLib.c

@@ -63,8 +63,6 @@ DebugPrint (
     return;
   }
 
-  Length = AsciiStrLen (Buffer);
-
   //
   // Convert the DEBUG() message to an ASCII String
   //
@@ -72,6 +70,8 @@ DebugPrint (
   AsciiVSPrint (Buffer, sizeof (Buffer), Format, Marker);
   VA_END (Marker);
 
+  Length = AsciiStrLen (Buffer);
+
   //
   // Send the print string to debug output handler
   //