screen editor -- fix STOP-DEL

[chitselb]

test procedure:

enter the editor, create several unique screens and page through them. Clear and update a few screens

• clear screen 0
• don't clear screen 0
• clear adjacent screens
• clear several nonadjacent screens individually and in groups
• clear alternating screens STOP-DEL STOP-HOME, STOP-? and STOP-UP to review the screens
• verify correct number of screens
• verify they're the original screens
• verify it works with data blocks
• verify it does not delete a data block consisting of 1024 spaces
• verify correct ordering
• verify no blank screens (other than screen 0)

[chitselb]

any screens that are blank are removed from VMBUF It may not remove screen 0 (the index screen)