Fix bug involving wraparound copy-and-paste to a blank line.

Identified by Anders Kaseorg.

Test case:

( reset; seq 100; printf 'Hello, world!'; sleep 1;
printf '\t\t\t\t\t\t\t\t\t\t   \r\e[K'; sleep 1; printf '\n' )
> /dev/pts/16 2>&1

(replace /dev/pts/16 with mosh or termemu terminal)