Cancelling character table dialogue in importer causes OpenLP to crash

See: http://support.openlp.org/issues/1833

"I was importing
songs from EasyWorship into LP. The import wizard starts to ask which
character table should be used.But after a few minutes, openLP asked me
it again and again and again....When I cancel this question, this report
function for errors appears."

After a bit of investigation, it appears that in the strip_rtf function we are not handling if the user cancels the QInputDialog in get_encoding.

For the problem reported, it looks likely that the user selected the wrong encoding.