Fix race condition issue in combineLatest

[AdamPitt7]

This is to fix a race condition in the combineLatest function.
I was getting nil pointer exceptions when using combineLatest callback function f FuncN with multiple observables which are also using combineLatest.

From what I can tell, the race condition is caused when one observable has incremented the counter, but has not yet set the cache variable s[i] = item.V
Meanwhile, another observable is checking if the counter is equal to the number of observables atomic.LoadUint32(&counter) == size and then calling the callback function with the cache variable which now includes a nil value.

By moving the counter increment into the lock, this should fix the race condition issue.