MDEV-19485: Crash on purge after ADD SPATIAL INDEX

row_build_spatial_index_key(): Return early if the column is missing
in the table row tuple.

This is a regression that was introduced by
commit 0e5a4ac2.

mysql-test/suite/innodb_gis/r/rtree_add_index.result

+SET @saved_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+CREATE TABLE t1 (g MULTIPOINT NOT NULL) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('');
+connect purge_control,localhost,root;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+DELETE FROM t1;
+ALTER TABLE t1 ADD SPATIAL INDEX (g);
+disconnect purge_control;
+InnoDB		0 transactions not purged
+DROP TABLE t1;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;

mysql-test/suite/innodb_gis/t/rtree_add_index.test

+--source include/have_innodb.inc
+
+SET @saved_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+
+CREATE TABLE t1 (g MULTIPOINT NOT NULL) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('');
+
+connect purge_control,localhost,root;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+
+DELETE FROM t1;
+
+ALTER TABLE t1 ADD SPATIAL INDEX (g);
+
+disconnect purge_control;
+--source ../../innodb/include/wait_all_purged.inc
+DROP TABLE t1;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;

storage/innobase/row/row0row.cc

@@ -62,6 +62,10 @@ static bool row_build_spatial_index_key(
 	ulint			flag,
 	mem_heap_t*		heap)
 {
+	if (dfield2->type.mtype == DATA_MISSING) {
+		return false;
+	}
+
 	double*			mbr;
 
 	dfield_copy(dfield, dfield2);
@@ -92,6 +96,7 @@ static bool row_build_spatial_index_key(
 	if (!dfield_is_ext(dfield2)) {
 		dptr = static_cast<const byte*>(dfield_get_data(dfield2));
 		dlen = dfield_get_len(dfield2);
+		ut_ad(dptr != &data_error);
 		goto write_mbr;
 	}