MdeModulePkg/EbcDebugger: Add ASSERT to ensure FieldBuffer is not NULL

In function EdbLoadCodBySymbolByIec(), AsciiStrGetNewTokenField() at line
1589 will return NULL if the first character in 'LineBuffer' is '\0'. But
the previous if statement at line 1576 ensures the above case will not
happen.

This commit adds ASSERT as warnings for the case that will not happen.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>