unix: fix memory leak when uv_loop_init() fails

`uv_signal_init()` leads to the allocation of an IO watcher,
so if the loop initialization fails at a later point,
the `loop->watchers` list needs to be released.

PR-URL: https://github.com/libuv/libuv/pull/2837
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

src/unix/loop.c

@@ -106,6 +106,8 @@ fail_rwlock_init:
 fail_signal_init:
   uv__platform_loop_delete(loop);
 
+  uv__free(loop->watchers);
+  loop->nwatchers = 0;
   return err;
 }