Fix plan cache issue in PL/pgSQL CALL

Fix plan cache issue in PL/pgSQL CALL

If we are not going to save the plan, then we need to unset expr->plan
after we are done, also in error cases.  Otherwise, we get a dangling
pointer next time around.

This is not the ideal solution.  It would be better if we could convince
SPI not to associate a cached plan with a resource owner, and then we
could just save the plan in all cases.  But that would require bigger
surgery.

Reported-by: Pavel Stehule <pavel.stehule@gmail.com>