Fix possible NULL dereference in ExecAlterObjectDependsStmt

Fix possible NULL dereference in ExecAlterObjectDependsStmt

I used the wrong variable here.  Doesn't make a difference today because
the only plausible caller passes a non-NULL variable, but someday it
will be wrong, and even today's correctness is subtle: the caller that
does pass a NULL is never invoked because of object type constraints.
Surely not a condition to rely on.

Noted by Coverity