plpgsql's exec_assign_value() freed the old value of a variable before

plpgsql's exec_assign_value() freed the old value of a variable before
copying/converting the new value, which meant that it failed badly on
"var := var" if var is of pass-by-reference type.  Fix this and a similar
hazard in exec_move_row(); not sure that the latter can manifest before
8.0, but patch it all the way back anyway.  Per report from Dave Chapeskie.