Preinitialize `workCancel` function in case `StopAndCancel` called before start

[brandur]

This one's related to #549. Although the most proximate problem in that repro code is that there was no error check when calling Start, it did reveal a legitimate problem in that the River client will panic in case StopAndCancel is called before Start because workCancel was never set.

Here, initialize workCancel in the client's constructor. During normal operation this will be overwritten almost immediately on Start as the client starts up, but in case Start was never called or didn't run successfully, it provides a function for StopAndCancel to call so that it doesn't panic.

Fixes #549.

[bgentry]

@brandur oh, probably deserves a changelog entry too

[brandur]

@brandur oh, probably deserves a changelog entry too

Added.