If you entered a multi-byte character into a prompt and then changed your mind
and used backspace to fix the character, only the last byte of the character was
removed. Because pango is intelligent, it noticed the broken utf8 and
complained.
So far nothing new. But since 711d78b50c the textbox will throw a lua error
when it gets an invalid text (= pango complains). Throwing an unprotected lua
error in this context causes the keygrabber to be killed which stops the prompt.
Fix this by removing bytes as long as there are bytes left that can be removed.
This is FS#801.
Signed-off-by: Uli Schlachter <psychon@znc.in>
26d97d0273