MDEV-21792 Server aborts upon attempt to create foreign key on spatial field

- mbmaxlen is always 0 for binary field. Ignore the assert of checking
field->prefix_len % field->col->mbmaxlen == 0.

mysql-test/suite/innodb/r/foreign_key.result

@@ -522,3 +522,7 @@ test.t1	check	status	OK
 DROP TABLE t1;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;
 # End of 10.2 tests
+CREATE TABLE t1 (a GEOMETRY, INDEX(a(8)),
+FOREIGN KEY (a) REFERENCES x (xx)) ENGINE=InnoDB;
+ERROR HY000: Can't create table `test`.`t1` (errno: 150 "Foreign key constraint is incorrectly formed")
+# End of 10.4 tests

mysql-test/suite/innodb/t/foreign_key.test

@@ -498,4 +498,12 @@ SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;
 
 --echo # End of 10.2 tests
 
+# MDEV-21792 Server aborts upon attempt to create foreign key on spatial field
+# Fail to create foreign key for spatial fields
+--error ER_CANT_CREATE_TABLE
+CREATE TABLE t1 (a GEOMETRY, INDEX(a(8)),
+		 FOREIGN KEY (a) REFERENCES x (xx)) ENGINE=InnoDB;
+
+-- echo # End of 10.4 tests
+
 --source include/wait_until_count_sessions.inc

storage/innobase/dict/dict0dict.cc

@@ -1839,7 +1839,8 @@ dict_index_add_to_cache(
 			   > field->col->max_prefix) {
 			/* Set the max_prefix value based on the
 			prefix_len. */
-			ut_ad(field->prefix_len % field->col->mbmaxlen == 0);
+			ut_ad(field->col->is_binary()
+			      || field->prefix_len % field->col->mbmaxlen == 0);
 			field->col->max_prefix = field->prefix_len;
 		}
 		ut_ad(field->col->ord_part == 1);

storage/innobase/include/dict0mem.h

@@ -2,7 +2,7 @@
 
 Copyright (c) 1996, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -733,6 +733,9 @@ struct dict_col_t{
 				 | CHAR_COLL_MASK << 16
 				 | DATA_LONG_TRUE_VARCHAR));
 	}
+
+  /** @return whether the column values are comparable by memcmp() */
+  inline bool is_binary() const { return prtype & DATA_BINARY_TYPE; }
 };
 
 /** Index information put in a list of virtual column structure. Index