MdeModulePkg/EbcDebugger: Add missing check for symbol not found

MdeModulePkg/EbcDebugger: Add missing check for symbol not found

In function DebuggerDisplaySymbolAccrodingToAddress(), when variable
'CandidateAddress' (returned by EbdFindSymbolAddress function) equals
(UINTN) -1, it also indicates that the symbol is not found at the given
address.

This commit adds this missing check.

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>