patch 9.1.0310: Filler lines not checked properly in get_scroll_overlap()

patch 9.1.0310: Filler lines not checked properly in get_scroll_overlap()

Problem:  Filler lines not checked properly in get_scroll_overlap().
Solution: Add missing parentheses (zeertzjq).

The missing parentheses causes the second argument to diff_check_fill()
to always be 0 as it is the result of a comparison between a positive
integer and -1 (the value of BACKWARD), in which case diff_check_fill()
always returns 0 instead of the number of filler lines above a line.

It's very hard to add a test for this, because this mistake at most
leads to 2 screen lines of difference in scrolling behavior, and in
cases where it may indeed lead to a difference in behavior, neither
behavior achieves complete symmetry between CTRL-F and CTRL-B.

closes: #14527

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>