patch 9.2.0443: GUI: cancelling save dialog overwrites or discards unnamed buffer

patch 9.2.0443: GUI: cancelling save dialog overwrites or discards unnamed buffer

Problem:  When closing gvim with an unsaved unnamed buffer, choosing
          "Yes" in the "Save changes?" dialog and then "Cancel" in the
          file selection dialog either silently writes the buffer to a
          file named "Untitled" (overwriting any existing file with
          that name) or discards the buffer altogether
          (vibs29, after v9.1.0265).
Solution: In dialog_changed(), if browse_save_fname() leaves the buffer
          without a file name, treat it as a cancel and return without
          saving.  Also stop clearing the modified flag in the restore
          path on write failure, so the unsaved changes are kept and
          the caller (e.g. gui_shell_closed()) can also cancel the
          close.  Pre-fill the file dialog with "Untitled" to match
          the preceding "Save changes to ..." prompt.  Add a test for
          the write-failure path (Hirohito Higashi).

fixes:  #20132
closes: #20143

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>