Add a missing check in uv__io_close() where it called
uv__platform_invalidate_fd() without checking that the
watcher actually has a valid file descriptor assigned.
Fixes: https://github.com/libuv/libuv/issues/2181
PR-URL: https://github.com/libuv/libuv/pull/2182
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>