In FcStrListCreate() we were increasing reference count of set,
however, if set had a const reference (which is the case for list
of languages), and with multiple threads, the const ref (-1) was
getting up to 1 and then a decrease was destroying the set.  Ouch.

Here's the valgrind error, which took me quite a few hours of
running to catch:

==4464== Invalid read of size 4
==4464==    at 0x4E58FF3: FcStrListNext (fcstr.c:1256)
==4464==    by 0x4E3F11D: FcConfigSubstituteWithPat (fccfg.c:1508)
==4464==    by 0x4E3F8F4: FcConfigSubstitute (fccfg.c:1729)
==4464==    by 0x4009FA: test_match (simple-pthread-test.c:53)
==4464==    by 0x400A6E: run_test_in_thread (simple-pthread-test.c:68)
==4464==    by 0x507EE99: start_thread (pthread_create.c:308)
==4464==  Address 0x6bc0b44 is 4 bytes inside a block of size 24 free'd
==4464==    at 0x4C2A82E: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4464==    by 0x4E58F84: FcStrSetDestroy (fcstr.c:1236)
==4464==    by 0x4E3F0C6: FcConfigSubstituteWithPat (fccfg.c:1507)
==4464==    by 0x4E3F8F4: FcConfigSubstitute (fccfg.c:1729)
==4464==    by 0x4009FA: test_match (simple-pthread-test.c:53)
==4464==    by 0x400A6E: run_test_in_thread (simple-pthread-test.c:68)
==4464==    by 0x507EE99: start_thread (pthread_create.c:308)

Thread test is running happily now.  Will add the test in a moment.