Fix simple_prompt again
Fix simple_prompt again

Although commit ca70e5e3 tried to fix simple_prompt code because the function
signature changed, it is still broken. :( Mehmet Yunus Tavas reported a crash
offlist. I should have allocated prompt_password if v10+ was used. I also fixed
an off-by-one error because \0 is counted as length in v10+ and it wasn't in
9.6.

eulerto/pgquarrel,eulerto/pgquarrel