[listview] fix jumping over very long lines with Page Up

[Yukigaru]

Hi, this fixes an issue with jumping over very long lines (that don't even fit on screen) (btw this's a common case in our logs) with "Page Up".
In such cases rows_available returns 1 and shift_amount evaluates to 0, meaning you can't progress towards the top. In the fix I ensured that rows_available has enough value to proceed, which is close to what has been done in "Page Down" code.

Note #1: I used this Python script to generate a text file to test on:

with open("test-big.log", "w") as f:
    f.writelines(f"before {i}\n" for i in range(1, 81))
    f.writelines(f"{i} ThisIsALongLine, " for i in range(1, 1500))
    f.write("\n")
    f.writelines(f"after {i}\n" for i in range(1, 81))

[tstack]

Thanks! I appreciate you providing the script to help with reproduction and the change seems to work.

Sorry for the trouble, I don't use word-wrap much, so don't feel the pain.