Add pointer check for NULL before dereference it.

Signed-off-by: ydong10
Reviewed-by: rsun3, lgao4

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12472 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/SetupBrowserDxe/Presentation.c

@@ -533,6 +533,7 @@ DisplayForm (
 
     if (!Suppress) {
       StringPtr = GetToken (Statement->Prompt, Handle);
+      ASSERT (StringPtr != NULL);
 
       Width     = GetWidth (Statement, Handle);