First the crash report:

```
  testInterruptedRecvmsgTimeout (__main__.InterruptedRecvTimeoutTest) ... Traceback (most recent call last):
    File "/home/travis/build/gevent/gevent/src/gevent/_ffi/loop.py", line 83, in python_callback
      def python_callback(self, handle, revents):
    File "/home/travis/build/gevent/gevent/src/greentest/3.6/test_socket.py", line 3698, in <lambda>
      lambda signum, frame: 1 / 0)
  ZeroDivisionError: division by zero
  Fri Dec 15 16:09:36 2017
  ok
  testInterruptedSendTimeout (__main__.InterruptedSendTimeoutTest) ... Fatal Python error: ffi.from_handle() detected that the address passed points to garbage. If it is really the result of ffi.new_handle(), then the Python object has already been garbage collected

  Thread 0x00002b26d4c01700 (most recent call first):
    File "/home/travis/build/gevent/gevent/src/gevent/_threading.py", line 152 in wait
    File "/home/travis/build/gevent/gevent/src/gevent/_threading.py", line 436 in get
    File "/home/travis/build/gevent/gevent/src/gevent/threadpool.py", line 200 in _worker

  Thread 0x00002b26d4a00700 (most recent call first):
    File "/home/travis/build/gevent/gevent/src/gevent/_threading.py", line 152 in wait
    File "/home/travis/build/gevent/gevent/src/gevent/_threading.py", line 436 in get
    File "/home/travis/build/gevent/gevent/src/gevent/threadpool.py", line 200 in _worker

  Current thread 0x00002b26cc7b2a00 (most recent call first):
    File "/home/travis/build/gevent/gevent/src/gevent/_ffi/loop.py", line 178 in python_stop
    File "/home/travis/build/gevent/gevent/src/gevent/libuv/loop.py", line 289 in run
    File "/home/travis/build/gevent/gevent/src/gevent/hub.py", line 688 in run
```

The main problem turned out to be the way in which libuv reports
signals at random times, much worse than the way libev does. This led
to callback functions unexpectedly returning 0, because that's the
default for an unhandled exception or an onerror handler that returns
None. That in turn led to calls to python_stop that we weren't
expecting using a handle that had already been deallocated.

The fix is to both be defensive about what handles we try to use, and
to be more explicit about our error returns.