If the modify callback function returned a pointer *within* the list item's
allocated buffer, the strcpy() would copy from potentialy free'd memory as
realloc may change the location of the heap buffer when resizing.
Fixed by allocating a copy of the returned pointer before freeing the original
list item buffer and then just assign the allocated pointer (no copying
needed). This likely will result in more heap fragmentation for modified list
items are now newly-allocated buffers rather than reallocated existing buffers
but the other option would have been to allocate a temporary copy of the string
before reallocating and then copying and that would've been a lot more overhead
than with this approach.

This likely fixes any crashes seen in recent revs of v3.17c (e.g. when
imported QWK or REP packets and text/*.can and the twitlist.cfg are parsed).
The sighting on Vertrauen was only in the Windows build and appeared when
importing QWK/REP packets.