Error code is inverted (which makes sense: who returns 'true' on
error?), and anyway there's a leak if we do error.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
`pay_index` has no valid value if not PAID anyway, so
we should correctly leave it uninitialized.
Analysis via valgrind will catch incorrect use of
uninitialized fields.
If we load it with a dummy 0 value, then an
incorrect use of `pay_index` whan invoice is not
PAID will not get caught by valgrind.
