Fix warnings: ex_cmds2.c: ex_listdo(): Np dereference: FP.

Problem    : Dereference of null pointer @ 1909.
Diagnostic : False positive.
Rationale  : Suggested path error first assumes tp == NULL and later one
             valid_tabpage(tp), which is not possible.
Resolution : Assert tp != NULL if valid_tabpage(tp).

src/nvim/ex_cmds2.c

@@ -1906,6 +1906,7 @@ void ex_listdo(exarg_T *eap)
         /* go to window "tp" */
         if (!valid_tabpage(tp))
           break;
+        assert(tp);
         goto_tabpage_tp(tp, TRUE, TRUE);
         tp = tp->tp_next;
       } else if (eap->cmdidx == CMD_bufdo) {