Fix low/high crash for big scrolloff values

[joelim-work]

• Fixes Crash on :low #1500

Steps to reproduce crashing for low:

1. Set scrolloff to a large value, for example set scrolloff 1000
2. Ensure the current directory has more files than the terminal height
3. Run the low command

Steps to reproduce crashing for high:

1. Set scrolloff to a large value, for example set scrolloff 1000
2. Scroll down (e.g. using scroll-down) so that the first file in the directory is not shown
3. Run the high command

This happens because the code does not cap the value of scrolloff at half the window height like the up/down commands.