Reapply a MySQL 5.6.23/5.7.10 Oracle Bug#20029625 fix that was inadvertently...

Reapply a MySQL 5.6.23/5.7.10 Oracle Bug#20029625 fix that was inadvertently reverted in MariaDB 10.2.2

The fix https://github.com/mysql/mysql-server/commit/716f97e2714e70f35d2dc01f0125ed833c62b408
was inadvertently reverted in
commit 2e814d47
"Merge InnoDB 5.7 from mysql-5.7.9".

Reapply the fix, because the test of the bug would fail
after merging MDEV-13838, which replaced an earlier incorrect
bug fix with a correct one.

storage/innobase/dict/dict0mem.cc

@@ -439,6 +439,9 @@ dict_mem_table_col_rename_low(
 	ut_ad(from_len <= NAME_LEN);
 	ut_ad(to_len <= NAME_LEN);
 
+	char from[NAME_LEN];
+	strncpy(from, s, NAME_LEN);
+
 	if (from_len == to_len) {
 		/* The easy case: simply replace the column name in
 		table->col_names. */
@@ -523,14 +526,54 @@ dict_mem_table_col_rename_low(
 
 		foreign = *it;
 
-		for (unsigned f = 0; f < foreign->n_fields; f++) {
-			/* These can point straight to
-			table->col_names, because the foreign key
-			constraints will be freed at the same time
-			when the table object is freed. */
-			foreign->foreign_col_names[f]
-				= dict_index_get_nth_field(
-					foreign->foreign_index, f)->name;
+		if (foreign->foreign_index == NULL) {
+			/* We may go here when we set foreign_key_checks to 0,
+			and then try to rename a column and modify the
+			corresponding foreign key constraint. The index
+			would have been dropped, we have to find an equivalent
+			one */
+			for (unsigned f = 0; f < foreign->n_fields; f++) {
+				if (strcmp(foreign->foreign_col_names[f], from)
+				    == 0) {
+
+					char** rc = const_cast<char**>(
+						foreign->foreign_col_names
+						+ f);
+
+					if (to_len <= strlen(*rc)) {
+						memcpy(*rc, to, to_len + 1);
+					} else {
+						*rc = static_cast<char*>(
+							mem_heap_dup(
+								foreign->heap,
+								to,
+								to_len + 1));
+					}
+				}
+			}
+
+			dict_index_t* new_index = dict_foreign_find_index(
+				foreign->foreign_table, NULL,
+				foreign->foreign_col_names,
+				foreign->n_fields, NULL, true, false,
+				NULL, NULL, NULL);
+			/* There must be an equivalent index in this case. */
+			ut_ad(new_index != NULL);
+
+			foreign->foreign_index = new_index;
+
+		} else {
+
+			for (unsigned f = 0; f < foreign->n_fields; f++) {
+				/* These can point straight to
+				table->col_names, because the foreign key
+				constraints will be freed at the same time
+				when the table object is freed. */
+				foreign->foreign_col_names[f]
+					= dict_index_get_nth_field(
+						foreign->foreign_index,
+						f)->name;
+			}
 		}
 	}
 
@@ -540,6 +583,8 @@ dict_mem_table_col_rename_low(
 
 		foreign = *it;
 
+		ut_ad(foreign->referenced_index != NULL);
+
 		for (unsigned f = 0; f < foreign->n_fields; f++) {
 			/* foreign->referenced_col_names[] need to be
 			copies, because the constraint may become