Tables have a 'lastfree' information only when needed

Only tables with some minimum number of entries in their hash part
have a 'lastfree' field, kept in a header before the node vector.

ltm.h

@@ -48,8 +48,8 @@ typedef enum {
 /*
 ** Mask with 1 in all fast-access methods. A 1 in any of these bits
 ** in the flag of a (meta)table means the metatable does not have the
-** corresponding metamethod field. (Bit 7 of the flag is used for
-** 'isrealasize'.)
+** corresponding metamethod field. (Bit 6 of the flag indicates that
+** the table is using the dummy node; bit 7 is used for 'isrealasize'.)
 */
 #define maskflags	(~(~0u << (TM_EQ + 1)))