Backport from 5.6: Bug#14769820 ASSERT FLEN == LEN

IN ALTER TABLE ... ADD UNIQUE KEY

A bogus debug assertion failure occurred when reporting a duplicate
key on a column prefix of a CHAR column.

This is a regression from Bug#14729221 IN-PLACE ALTER TABLE REPORTS ''
INSTEAD OF REAL DUPLICATE VALUE FOR PREFIX KEYS. The assertion is only
present when UNIV_DEBUG is defined (which it is in debug builds
starting from MySQL 5.5). It is a case of overasserting.

Fix approved by Inaam Rana on IM.

storage/innodb_plugin/handler/handler0alter.cc

@@ -108,13 +108,17 @@ innobase_col_to_mysql(
 		/* These column types should never be shipped to MySQL. */
 		ut_ad(0);
 
-	case DATA_CHAR:
 	case DATA_FIXBINARY:
 	case DATA_FLOAT:
 	case DATA_DOUBLE:
 	case DATA_DECIMAL:
 		/* Above are the valid column types for MySQL data. */
 		ut_ad(flen == len);
+		/* fall through */
+	case DATA_CHAR:
+		/* We may have flen > len when there is a shorter
+		prefix on a CHAR column. */
+		ut_ad(flen >= len);
 #else /* UNIV_DEBUG */
 	default:
 #endif /* UNIV_DEBUG */