Repair oversights in the mechanism used to store compiled plpgsql functions.

The original coding failed (tried to access deallocated memory) if there were
two active call sites (fn_extra pointers) for the same function and the
function definition was updated.  Also, if an update of a recursive function
was detected upon nested entry to the function, the existing compiled version
was summarily deallocated, resulting in crash upon return to the outer
instance.  Problem observed while studying a bug report from Sergiy
Vyshnevetskiy.

Bug does not exist before 8.1 since older versions just leaked the memory of
obsoleted compiled functions, rather than trying to reclaim it.