MdeModulePkg: Check the case caused by mismatch

MdeModulePkg: Check the case caused by mismatch

When mismatch happens,there exists one case that exit current
form and display last form.Assert code don't cover this case.
Now add check to handle this situation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18053 6f19259b-4bc3-4df7-8a09-765794883524