MDEV-7367: Updating a virtual column corrupts table which crashes server

Analysis: MySQL table definition contains also virtual columns. Similarly,
index fielnr references MySQL table fields. However, InnoDB table definition
does not contain virtual columns. Therefore, when matching MySQL key fieldnr
we need to use actual column name to find out referenced InnoDB dictionary
column name.

Fix: Add new function to match MySQL index key columns to InnoDB dictionary.

storage/xtradb/include/row0merge.h

@@ -95,6 +95,7 @@ struct index_field_t {
 	ulint		col_no;		/*!< column offset */
 	ulint		prefix_len;	/*!< column prefix length, or 0
 					if indexing the whole column */
+	const char*	col_name;	/*!< column name or NULL */
 };
 
 /** Definition of an index being created */