row_sel_field_store_in_mysql_format(): Do not pad the unused part of
the buffer reserved for a True VARCHAR column (introduced in 5.0.3).
Add Valgrind instrumentation ensuring that the unused part will be
flagged uninitialized.

row_sel_copy_cached_field_for_mysql(): New function: Copy a field
that is in the MySQL row format, not copying the unused tail of
VARCHAR columns.

row_sel_pop_cached_row_for_mysql(): Invoke
row_sel_copy_cached_field_for_mysql() for copying fields.
When the row is long, copy it field-by-field.

rb:715 approved by Inaam Rana