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

Log Message:
-----------
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.

Tags:
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REL7_4_STABLE

Modified Files:
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    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.93.2.1 -> r1.93.2.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.93.2.1&r2=1.93.2.2)