contains ONLY_FULL_GROUP_BY

The partitioning code needs to issue a Item::fix_fields()
on the partitioning expression in order to prepare 
it for being evaluated.
It does this by creating a special table and a table list 
for the scope of the partitioning expression.
But when checking ONLY_FULL_GROUP_BY the 
Item_field::fix_fields() was relying that there always be
cached_table set and was trying to use it to get the 
select_lex of the SELECT the field's table is in.
But the cached_table was not set by the partitioning code
that creates the artificial TABLE_LIST used to resolve the
partitioning expression and this resulted in a crash.
 
Fixed by rectifying the following errors :
1. Item_field::fix_fields() : the code that check for 
ONLY_FULL_GROUP_BY relies on having tables with 
cacheable_table set. This is mostly true, the only 
two exceptions being the partitioning context table
and the trigger context table.
Fixed by taking the current parsing context if no pointer
to the TABLE_LIST instance is present in the cached_table.

2. fix_fields_part_func() : 

2a. The code that adds the table being created to the 
scope for the partitioning expression is mostly a copy 
of the add_table_to_list and friends with one exception :
it was not marking the table as cacheable (something that
normal add_table_to_list is doing). This caused the 
problem in the check for ONLY_FULL_GROUP_BY in 
Item_field::fix_fields() to appear.
Fixed by setting the correct members to make the table
cacheable.
The ideal structural fix for this is to use a unified 
interface for adding a table to a table list 
(add_table_to_list?) : noted in a TODO comment

2b. The Item::fix_fields() was called with a NULL destination
pointer. This causes uninitalized memory reads in the 
overloaded ::fix_fields() function (namely 
Item_field::fix_fields()) as it expects a non-zero pointer 
there. Fixed by passing the source pointer similarly to how 
it's done in JOIN::prepare().