virpcivpdtest: Fix potential double-free()

virpcivpdtest: Fix potential double-free()

Inside the testPCIVPDResourceCustomCompareIndex() function we
have two variables @a and @b, both marked as g_autoptr(). Then,
towards the end of the function b->value is freed and set to
a->value. This is to make sure
virPCIVPDResourceCustomCompareIndex() works correctly even if
->value member is the same for both arguments.

Nevertheless, if the function returns anything else than 0 then
the control executes subsequent return statement and since
b->value points to the very same string as a->value a double free
will occur. Avoid this by setting b->value to NULL explicitly,
just like we are already doing for the successful path.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>