Backport from mysql-5.5 to mysql-5.1

Bug# 19699237: UNINITIALIZED VARIABLE IN
               ITEM_FIELD::STR_RESULT LEADS TO INCORRECT
               BEHAVIOR

ISSUE:
------
When the following conditions are satisfied in a query, a
server crash occurs:
a) Two rows are compared using a NULL-safe equal-to operator.
b) Each of these rows belong to different charsets.

SOLUTION:
---------
When one charset is converted to another for comparision,
the constructor of "Item_func_conv_charset" is called.
This will attempt to use the Item_cache if the string is a
constant. This check succeeds because the "used_table_map"
of the Item_cache class is never set to the correct value.
Since it is mistakenly assumed to be a constant, it tries
to fetch the relevant null value related fields which are
yet to be initialized. This results in valgrind issues
and wrong results.

The fix is to update the "used_table_map" of "Item_cache".
This will allow "Item_func_conv_charset" to realise that
this is not a constant.