libuv: context creation fails via destroy

When using a foreign libuv loop, context creation may fail after adding
handles to the foreign loop... if so, it can no longer deal with the
fatal error by unpicking the created context and returning NULL... it
has to brazen it out with a half-baked context that has already started
the destroy flow and allow the foreign loop to close out the handles
the usual way for libuv.

https://github.com/warmcat/libwebsockets/issues/2129