handle QP lookups involving escaped characters better

handle QP lookups involving escaped characters better

in QP keys, characters that are not common in DNS names are
encoded as two-octet sequences. this caused a glitch in iterator
positioning when some lookups failed.

consider the case where we're searching for "\009" (represented
in a QP key as {0x03, 0x0c}) and a branch exists for "\000"
(represented as {0x03, 0x03}). we match on the 0x03, and continue
to search down. at the point where we find we have no match,
we need to pop back up to the branch before the 0x03 - which may
be multiple levels up the stack - before we position the iterator.