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

Problem    : Dereference of null pointer @ 1903.
Diagnostic : False positive.
Rationale  : Suggested error path first assumes wp == NULL and later one
             win_valid(wp), which is not possible.
Resolution : Assert wp != NULL if win_valid(wp).

src/nvim/ex_cmds2.c

@@ -1897,6 +1897,7 @@ void ex_listdo(exarg_T *eap)
         /* go to window "wp" */
         if (!win_valid(wp))
           break;
+        assert(wp);
         win_goto(wp);
         if (curwin != wp)
           break;            /* something must be wrong */