MdeModulePkg/EbcDebugger: Add check for Entry and RetEntry

Entry and RetEntry might be NULL before used.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>

MdeModulePkg/Universal/EbcDxe/EbcDebugger/EdbSymbol.c

@@ -2062,7 +2062,7 @@ EdbPrintSource (
                     &RetObject,
                     &RetEntry
                     );
-  if (SymbolAddress == 0) {
+  if (SymbolAddress == 0 || RetEntry == NULL) {
     return 0 ;
   }