methods/ftp: Cope with weird PASV responses

methods/ftp: Cope with weird PASV responses

wu-ftpd sends the response without parens, whereas we expect
them.

I did not test the patch, but it should work. I added another
return true if Pos is still npos after the second find to make
sure we don't add npos to the string.

Thanks: Lukasz Stelmach for the initial patch
Closes: #420940
(cherry picked from commit 25a694165ae46c159e0d91bf0b27717f00dbc66e)