Fix: __powerline_last_status_prompt to handle unset argument safely

[manandhar01]

Summary

This PR fixes a small bug in the __powerline_last_status_prompt function where
an error would occur if the function was called without an argument.

Changes

• Replaced ${1?} with ${1:-0} to provide a safe default value when $1 is unset.
• This ensures that the function no longer throws an error if no argument is passed.

Why

The previous implementation used ${1?}, which causes error to show up before each prompt
if $1 is not set. This could break the prompt in certain cases. By switching to
${1:-0}, the function now defaults to 0 and continues safely.

Testing

• Verified that the function behaves correctly when called with and without arguments.
• Prompt now displays the correct status without errors.

[seefood]

seems legit. how did it work this far?

[manandhar01]

seems legit. how did it work this far?

No idea. It was working fine on my old system. Installed this on my new system yesterday and the error started popping up. At first I thought there was something wrong with my bashrc but it was fine. I then followed the code and ended up here.