Prevent c-treeACE SQL Shutdown Fault

c-treeACE SQL could terminate with a SIGSEGV exception during a seemingly normal shutdown sequence.

When shutting down, c-treeACE SQL checks an internal counter to determine if any SQL threads are still active. If threads remain active, the server does not free memory allocated by c-tree when it shuts down (to prevent the active threads from crashing). However, it was found that in a recent code line a SQL connection does not properly increment the counter. To prevent the invalid count, SQL threads now correctly increment the active thread counter when logging on and decrement the active thread counter when logging off.