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

Problem    : Dereference of null pointer @ 4512.
Diagnostic : False positive.
Rationale  : Suggested error path implies `fr == NULL` after 4504.
             That's not possible, because:
             - curfr and curfr->next must be both nonnull, as we are
               dragging the divider between the two.
             - after conditional, fr is one of those two (the one that
               grows).
Resolution : Assert fr.

src/nvim/window.c

@@ -4504,6 +4504,7 @@ void win_drag_vsep_line(win_T *dragwin, int offset)
       room += fr->fr_width - frame_minwidth(fr, NULL);
     fr = curfr;                         /* put fr at window that grows */
   }
+  assert(fr);
 
   if (room < offset)            /* Not enough room */
     offset = room;              /* Move as far as we can */