Small fix for _valueCopy()

Fernando Nasser <fnasser@redhat.com> writes:

Protect against a T_String Value (or other pointer Value) Value
where the pointer is NULL.
If we don't do it, _copyVariableSet() dumps core.

I believe that this change and the corresponding one in equalfuncs
should not be applied. A Value struct containing a null string
pointer is not a valid node. (The counterexample showing why
it should not be considered a useful representation of null-ness
is that there'd be no equivalent representation for null integer
Values.)

As with ConstraintsSetStmt, the correct fix is to change the
representation of a SET foo TO DEFAULT command. Thomas indicated
some interest in handling this detail; if he doesn't get around to
it, I will, or you can.

regards, tom lane