Fix warnings: window.c: tabline_height(): Np dereference: FP.

Problem    : Dereference of null pointer @ 4978.
Diagnostic : False positive.
Rationale  : tabline_height() shouldn't be called when a tab doesn't
             exist yet (this is, before initialization).
Resolution : Assert function precondition.

src/nvim/window.c

@@ -4976,6 +4976,7 @@ static void last_status_rec(frame_T *fr, int statusline)
  */
 int tabline_height(void)
 {
+  assert(first_tabpage);
   switch (p_stal) {
   case 0: return 0;
   case 1: return (first_tabpage->tp_next == NULL) ? 0 : 1;