grpc-js: Fix commitCallWithMostMessages trying to commit completed attempts

[murgatroid99]

This fixes #2347. The call site of commitCallWithMostMessages assumes that some call has been committed after the function returns. This can fail for two reasons:

1. All existing underlying calls are completed.
2. A completed call has more messages sent than all active calls

This change removes the second possibility entirely, and adds checks to handle the first case properly.