pgsql: Disallow deletion of CurrentExtensionObject while running extens

Disallow deletion of CurrentExtensionObject while running extension script.

While the deletion in itself wouldn't break things, any further creation
of objects in the script would result in dangling pg_depend entries being
added by recordDependencyOnCurrentExtension(). An example from Phil
Sorber convinced me that this is just barely likely enough to be worth
expending a couple lines of code to defend against. The resulting error
message might be confusing, but it's better than leaving corrupted catalog
contents for the user to deal with.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ec3e183ec51bb2d4032be6d5402ddc7b4c8acb71

Modified Files
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src/backend/commands/extension.c | 17 +++++++++++++++++
1 files changed, 17 insertions(+), 0 deletions(-)