patch 9.1.1455: Haiku: dailog objects created with no reference

patch 9.1.1455: Haiku: dailog objects created with no reference

Problem:  Haiku: dailog objects created with no reference
Solution: delete the objects before returning (jinyaoguo)

In the functions gui_mch_dialog() and gui_mch_font_dialog(), Dialog
objects are created but never escape the function scope. The call to
dialog->Go() only returns a boolean value and does not retain any
reference to the Dialog object itself, which may lead to potential
memory leak.

Fix this by deleting the object after using it.

closes: #17501

Signed-off-by: jinyaoguo <guo846@purdue.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>