OFConnectionManager: fix clang-analyzer warning

The analyzer wasn't convinced that cur_node would be non-NULL (and that the iov
would be initialized) if left > 0. I don't think there's an actual bug here,
but better to be safe.
OFStateManager: fix clang-analyzer warning

The analyzer assumed that the return value from ft_entry_set_effects could be
greater than zero. If this happened the entry would not be returned by
ft_entry_create, but ft_add would think it was returned.
Merge into master from pull request #135:
fix clang-analyzer warnings (https://github.com/floodlight/indigo/pull/135)