Fix some potential null pointer crashes

src/checksecurity.cpp

@@ -41,9 +41,15 @@ void CheckSecurity::readnum()
             tok = tok->next();
         }
 
+        if (!tok)
+            break;
+
         // Skip ahead a little with tok..
         tok = tok->tokAt(2);
 
+        if (!tok)
+            break;
+
         // Now take a look at the variable usage..
         if (varId == 0)
             continue;