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.

src/backend/commands/extension.c

@@ -1642,6 +1642,23 @@ RemoveExtensionById(Oid extId)
 	HeapTuple	tuple;
 	ScanKeyData entry[1];
 
+	/*
+	 * Disallow deletion of any extension that's currently open for insertion;
+	 * else subsequent executions of recordDependencyOnCurrentExtension()
+	 * could create dangling pg_depend records that refer to a no-longer-valid
+	 * pg_extension OID.  This is needed not so much because we think people
+	 * might write "DROP EXTENSION foo" in foo's own script files, as because
+	 * errors in dependency management in extension script files could give
+	 * rise to cases where an extension is dropped as a result of recursing
+	 * from some contained object.  Because of that, we must test for the case
+	 * here, not at some higher level of the DROP EXTENSION command.
+	 */
+	if (extId == CurrentExtensionObject)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("cannot drop extension \"%s\" because it is being modified",
+						get_extension_name(extId))));
+
 	rel = heap_open(ExtensionRelationId, RowExclusiveLock);
 
 	ScanKeyInit(&entry[0],