pgsql: Repair oversights in the mechanism used to store compiled plpgsql

Log Message:
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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.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/pl/plpgsql/src:
pl_comp.c (r1.94.2.2 -> r1.94.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c.diff?r1=1.94.2.2&r2=1.94.2.3)
pl_handler.c (r1.26 -> r1.26.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_handler.c.diff?r1=1.26&r2=1.26.2.1)
plpgsql.h (r1.65.2.2 -> r1.65.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.65.2.2&r2=1.65.2.3)