Fix memory leak in fts_flatcurve_xapian_query_iter_next()

[edieterich]

This patch fixes a memory leak in fts_flatcurve_xapian_query_iter_next().

valgrind:

==29989== 39,883 (256 direct, 39,627 indirect) bytes in 1 blocks are definitely lost in loss record 42 of 43
==29989==    at 0x4840F2F: operator new(unsigned long) (vg_replace_malloc.c:422)
==29989==    by 0x7E8B2B0: ??? (in /usr/lib/x86_64-linux-gnu/libxapian.so.30.12.3)
==29989==    by 0x7D930FD: Xapian::Enquire::Internal::get_mset(unsigned int, unsigned int, unsigned int, Xapian::RSet const*, Xapian::MatchDecider const*) const (in /usr/lib/x86_64-linux-gnu/libxapian.so.30.12.3)
==29989==    by 0x7D93363: Xapian::Enquire::get_mset(unsigned int, unsigned int, unsigned int, Xapian::RSet const*, Xapian::MatchDecider const*) const (in /usr/lib/x86_64-linux-gnu/libxapian.so.30.12.3)
==29989==    by 0x55940C4: fts_flatcurve_xapian_query_iter_next (fts-backend-flatcurve-xapian.cpp:1752)
==29989==    by 0x559457A: fts_flatcurve_xapian_run_query (fts-backend-flatcurve-xapian.cpp:1811)
==29989==    by 0x558DB9B: fts_backend_flatcurve_lookup_multi (fts-backend-flatcurve.c:593)
==29989==    by 0x558DEFA: fts_backend_flatcurve_lookup (fts-backend-flatcurve.c:652)
==29989==    by 0x536B35C: fts_backend_lookup (fts-api.c:377)
==29989==    by 0x5370156: fts_search_lookup_level_single (fts-search.c:53)
==29989==    by 0x5370156: fts_search_lookup_level (fts-search.c:216)
==29989==    by 0x53708EB: fts_search_try_lookup (fts-search.c:371)
==29989==    by 0x53708EB: fts_search_lookup (fts-search.c:383)
==29989==    by 0x53734CF: fts_mailbox_search_init (fts-storage.c:264)