Check after errors of SPI_execute() in xml.c

SPI gets used to build a list of relation OIDs for XML object
generation, and one code path building a list uses SPI_execute() without
looking at errors it produces.  So fix that.

Author: Mark Dilger
Reviewed-by: Michael Paquier, Pavel Stehule
Discussion: https://postgr.es/m/17d30445-4862-7917-170f-84328dcd292d@gmail.com

src/backend/utils/adt/xml.c

@@ -2461,8 +2461,12 @@ query_to_oid_list(const char *query)
 {
 	uint64		i;
 	List	   *list = NIL;
+	int			spi_result;
 
-	SPI_execute(query, true, 0);
+	spi_result = SPI_execute(query, true, 0);
+	if (spi_result != SPI_OK_SELECT)
+		elog(ERROR, "SPI_execute returned %s for %s",
+			 SPI_result_code_string(spi_result), query);
 
 	for (i = 0; i < SPI_processed; i++)
 	{