Assertion `fixed == 1' failed

(also fixes duplicate bug 57515)

agg_item_set_converter() (item.cc) handles conversion of 
character sets by creating a new Item. fix_fields() is then 
called on this newly created item. Prior to this patch, it was
not checked whether fix_fields() was successful or not. Thus, 
agg_item_set_converter() would return success even when an 
error occured. This patch makes it return error (TRUE) if 
fix_fields() fails.