Make pgstatindex respond to cancel interrupts.

A similar problem for pgstattuple() was fixed in April of 2010 by commit
33065ef8, but pgstatindex() seems to have
been overlooked.

Back-patch all the way, as with that commit, though not to 7.4 through
8.1, since those are now EOL.

contrib/pgstattuple/pgstatindex.c

@@ -156,6 +156,8 @@ pgstatindex(PG_FUNCTION_ARGS)
 		Page		page;
 		BTPageOpaque opaque;
 
+		CHECK_FOR_INTERRUPTS();
+
 		/* Read and lock buffer */
 		buffer = ReadBuffer(rel, blkno);
 		LockBuffer(buffer, BUFFER_LOCK_SHARE);