Fix calculations of whether to write updated child indexes.
Fix calculations of whether to write updated child indexes.

The new checks compare incremented values.  This is done so that the
sentinel value ^uint32(0), when incremented, wraps around to 0 and
when compared against non-zero child indexes, the check will determine
that the update to increase to child index should be performed.

decred/dcrwallet,jrick/btcwallet,jrick/btcwallet,jrick/dcrwallet,jrick/dcrwallet,decred/dcrwallet