MdeMoudlePkg/DisplayEngine: Fix incorrect index used in array "InputText"

MdeMoudlePkg/DisplayEngine: Fix incorrect index used in array "InputText"

When set value to the array "InputText", the index was used incorrectly.
And the array "InputText" is not initialized. These will cause some value
in the array is random, so it will be shown incorrectly sometimes.
This patch is to fix this issue.

https://bugzilla.tianocore.org/show_bug.cgi?id=358

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Wang Cloud <winggundum82@163.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>