Bug#59699 - Hidden bug would cause a crash for unknown tablespace flags.

The bug would cause a crash of InnoDB if a non-standard or unknown table
flags existed in a SYS_TABLES record. This is important because the next
file version, Cheetah, will identify itself by expanding this field.  So
unless this is fixed, an older engine that tries to open a table in a
tablespace with a newer file version will crash instead of report an error
and refuse to open the table, as it should do.

Reviewed at RB://583.    Approved by Marko.

storage/innobase/dict/dict0load.c

@@ -1552,7 +1552,7 @@ err_len:
 				"InnoDB: in InnoDB data dictionary"
 				" has unknown type %lx.\n",
 				(ulong) flags);
-			return(NULL);
+			return("incorrect flags in SYS_TABLES");
 		}
 	} else {
 		flags = 0;