fixed NULL check from getNextEntry

src/symbol_table.c

@@ -1221,7 +1221,17 @@ ListOfTable *getRestOfChildren(ListOfTable *lt) { return lt->next; }
 TableNode *getFirstEntry(SymbolTable *st) { return st->entries; }
 
 // Segfaults when passed an invalid table node!
-TableNode *getNextEntry(TableNode *tn) { return tn; }
+TableNode *getNextEntry(TableNode *tn) {
+                if (tn == NULL) {
+                printdebug("passed a NULL table node to getNextEntry");
+                return undefined;
+                }
+                if (tn == undefined) {
+                printdebug("passed an undefined table node to getNextEntry");
+                return undefined;
+                }
+                return tn->next; 
+        }
 
 
 // uncomment the below main function along with the headers above for a simple