check if cb is defined before calling in destroy

[DBC201]

When req.session.destroy() is called without any parameter in express, cb in destroy() throws an error since it is not defined.

[TimDaub]

While this change technically is fine because it technically doesn't touch any functionality, @DBC201, I'm wondering where it is the case that function destroy is called without passing a callback function.

A few data points, why I'm confused:

• The express-session interface specification makes no mention of cb being undefined at times: https://github.com/expressjs/session#session-store-implementation
• However, on the contrary, https://github.com/tj/connect-redis handles the possibility of cb not being callable by introducing a "noop" function: https://github.com/tj/connect-redis/blob/a4770bd07f7a248beb902708b3c1d3914ddb5dc6/lib/connect-redis.js#L12 and then making it cb's default value.
• I'll merge your PR, open an issue upstream to clarify the specs, and try to update everything accordingly.

[DBC201]

jdesboeufs/connect-mongo#272

mongo also does something similar, however I agree with your point that the docs say it's required.