small optimisation in signed_literal


sql/field.cc:
  layout fixed
sql/item.cc:
  DBUG_ASSERT(fixed == 1); added to val*
  layout fixed
  fixed= 1; added where it was forgoten in fix_fields
  Item_string can be used without fix_fields
sql/item.h:
  DBUG_ASSERT(fixed == 1); added to val*
  Item_string can be used without fix_fields
sql/item_cmpfunc.cc:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_cmpfunc.h:
  fixed layout and getting Item statistic
sql/item_func.cc:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_func.h:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_geofunc.cc:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_strfunc.cc:
  DBUG_ASSERT(fixed == 1); added to val*
  layout fixed
sql/item_strfunc.h:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_subselect.cc:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_sum.cc:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_sum.h:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_timefunc.cc:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_timefunc.h:
  DBUG_ASSERT(fixed == 1); added to val*
sql/item_uniq.h:
  DBUG_ASSERT(fixed == 1); added to val*
sql/sql_base.cc:
  Item creation revised
sql/sql_help.cc:
  Item creation revised
sql/sql_load.cc:
  Item creation revised
sql/sql_parse.cc:
  fix_field call added
sql/sql_select.cc:
  Item creation revised
sql/sql_show.cc:
  Item creation revised
sql/sql_union.cc:
  Item creation revised
sql/sql_update.cc:
  Item creation revised
sql/sql_yacc.yy:
  Item creation revised
  small optimisation in signed_literal