MdeModulePkg/HiiDatabase: clean the value before setting default string

MdeModulePkg/HiiDatabase: clean the value before setting default string

For string op-code, the default string may not reach the
maximum size, so when generating <AltResp> string, we should
clean the value before setting the default string.

https://bugzilla.tianocore.org/show_bug.cgi?id=375

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.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>