sd-event: fix fd leak when fd is owned by IO event source

sd-event: fix fd leak when fd is owned by IO event source

When an IO event source owns relevant fd, replacing with a new fd leaks
the previously assigned fd.
===
sd_event_add_io(event, &s, fd, ...);
sd_event_source_set_io_fd_own(s, true);
sd_event_source_set_io_fd(s, new_fd);  <-- The previous fd is not closed.
sd_event_source_unref(s);  <-- new_fd is closed as expected.
===

Without the change, valgrind reports the leak:
==998589==
==998589== FILE DESCRIPTORS: 4 open (3 std) at exit.
==998589== Open file descriptor 4:
==998589==    at 0x4F119AB: pipe2 (in /usr/lib64/libc.so.6)
==998589==    by 0x408830: test_sd_event_source_set_io_fd (test-event.c:862)
==998589==    by 0x403302: run_test_table (tests.h:171)
==998589==    by 0x408E31: main (test-event.c:935)
==998589==
==998589==
==998589== HEAP SUMMARY:
==998589==     in use at exit: 0 bytes in 0 blocks
==998589==   total heap usage: 33,305 allocs, 33,305 frees, 1,283,581 bytes allocated
==998589==
==998589== All heap blocks were freed -- no leaks are possible
==998589==
==998589== For lists of detected and suppressed errors, rerun with: -s
==998589== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)