IntelFrameworkModulePkg/DebugLib: Fix string copy issue

IntelFrameworkModulePkg/DebugLib: Fix string copy issue

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1826

There is a bug to use AsciiStrCpyS to copy a truncated
string. If would cause an assert because the truncated
length is always less than the source string length. It
should use the AsciiStrnCpyS instead.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>