MB-12251: Use original MBB for dimensionality comparison

The key of the root node can be be unset. Hence use the original MBB
for the comparison between the query and the index dimensionality.
The original MBB is always set and has the same dimensionality the
index has.

Change-Id: I9b33219fa1640d126013704899fa8c3457810530
Reviewed-on: http://review.couchbase.org/42756
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>