Add the missing check for NULL pointer before use it.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10116 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/SetupBrowserDxe/Ui.h

@@ -623,6 +623,8 @@ ClearLines (
   hit, then 2 Unicode character will consume an output storage
   space with size of CHAR16 till a NARROW_CHAR is hit.
 
+  If String is NULL, then ASSERT ().
+
   @param String          The input string to be counted.
 
   @return Storage space for the input string.