Fix LP BUG#719198, LP BUG#730604

Analysis (BUG#719198):
The assert failed because the execution code for
partial matching is designed with the assumption that
NULLs on the left side are detected as early as possible,
and a NULL result is returned before any lookups are
performed at all.

However, in the case of an Item_cache object on the left
side, null was not detected properly, because detection
was done via Item::is_null(), which is not implemented at
all for Item_cache, and resolved to the default Item::is_null()
which always returns FALSE.

Solution:
Imlpement Item::is_null().

******

Analysis (BUG#730604):
The method Item_field::is_null() determines if an item is NULL from its
Item_field::field object. However, for Item_fields that represent internal
temporary tables, Item_field::field represents the field of the original
table that was the source for the temporary table (in this case t1.f3).
Both in the committed test case, and in the original bug report the current
value of t1.f3 is not NULL. This results in an incorrect count of NULLs
for this column. As a consequence, all related Ordered_key buffers are
allocated with incorrect sizes. Depending on the exact query and data,
these incorrect sizes result in various crashes or failed asserts.

Solution:
The correct value of the current field of the internal temp table is
in Item_field::result_field. This value is determined by
Item::is_null_result().

sql/sql_class.cc

@@ -3102,7 +3102,7 @@ bool select_materialize_with_stats::send_data(List<Item> &items)
 
   while ((cur_item= item_it++))
   {
-    if (cur_item->is_null())
+    if (cur_item->is_null_result())
     {
       ++cur_col_stat->null_count;
       cur_col_stat->max_null_row= count_rows;