When EN_Destructor is called it searches through the connection array

When EN_Destructor is called it searches through the connection array
and destroys any connections which belong that environment. Unfortunately
it doesn't NULL out the connection array entry afterwards so a future
call to EN_Destructor can wind up trying to destroy that connection
again if another environment has been allocated at the same address.

Also, shouldn't EN_Destructor hold the CONNS critical section while it
is playing with the connection array?

The attached patch fixes both of these issues and seems to have fixed
the crashes I was observing.
[Tom Hughes]