Field_bit::pack() and Field_bit::unpack() does not work correctly

Fixing code for Field_bit packing and unpacking to work with arbitrary
pointers instead of requiring Field::ptr


sql/field.cc:
  Fixing Field_bit::pack() and Field_bit::unpack() so that they accept
  an arbitrary pointer to pack/unpack to/from.
sql/sql_class.cc:
  Removing unneeded move_field_offset() nefore packing field.