add some error check and return.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4912 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/BdsDxe/MemoryTest.c

@@ -152,6 +152,8 @@ Returns:
                           SizeOfY - (PosY - GLYPH_HEIGHT - 1),
                           SizeOfX * sizeof (EFI_UGA_PIXEL)
                           );
+    } else {
+      return EFI_UNSUPPORTED;
     }
   }
   //
@@ -185,6 +187,8 @@ Returns:
                           BlockHeight,
                           (BlockWidth) * sizeof (EFI_UGA_PIXEL)
                           );
+    } else {
+      return EFI_UNSUPPORTED;
     }
   }