Commit a2d28010cc ("cleanup: find_subfiles.c: simplify (mainly using
bstr)") was missing a bstrdup() in subtitle search code, and thus the
code erroneously modified the original filename string passed in. As a
result anything which printed or otherwise used the filename after
that would use a lowercase-converted version instead of the actual
file name. Fix by adding the bstrdup() to operate on a local copy of
the name instead.