scrolling: don't overscroll when the edit window has just one row

If the edit window consists of a single row, then the do_down() call
has already brought the desired line or chunk into view -- no attempt
should then be made to scroll the screen further.

This fixes https://savannah.gnu.org/bugs/?56553.

Bug existed since version 2.9.6, commit 0d9080a2.

src/move.c

@@ -554,12 +554,12 @@ void do_scroll_down(void)
 	if (openfile->current_y == 0)
 		do_down();
 
-	if (openfile->edittop->next != NULL
+	if (editwinrows > 1 && (openfile->edittop->next != NULL
 #ifndef NANO_TINY
 				|| chunk_for(openfile->firstcolumn, openfile->edittop) <
 					number_of_chunks_in(openfile->edittop)
 #endif
-										)
+										))
 		edit_scroll(FORWARD);
 }
 #endif