pybind/rados: Fix error handling and leaks in aio

pybind/rados: Fix error handling and leaks in aio

aio_read:
The reference to ret_s begins existing at PyBytes_FromStringAndSize and
is handed over to the callback if rados_aio_read succeeds. This creates
a lot of subtle scenarios where it might not be XDECREFed (e.g. if
a KeyboardInterrupt arrives at the wrong time). Instead, store the pointer
to that buffer in the completion object, and hand over responsibility for
the XDECREF to it. This guarantees that the "special" reference to this
incomplete object will be released when the completion object is
deallocated.

Also make sure we don't try to _PyBytes_Resize with a negative length.

Add a failure case to the aio_read test in test_rados.py

Completion: the wrapper methods weren't being called, which prevents
the completion objects from being freed until the Ioctx is. Fix this
and add a refcount check to the aio_read test.

Signed-off-by: Hector Martin <marcan@marcan.st>