Fix error in Blob::operator==

Fix error in Blob::operator==

Fix typo in Blob::operaror== - incorrect comparision of
extMetaLen. Introduced by http://review.couchbase.org/69477

Bug identified by Coverity:

    CID 153111:  Incorrect expression  (CONSTANT_EXPRESSION_RESULT)
    "lhs.extMetaLen == lhs.extMetaLen" is always true regardless of
    the values of its operands because those operands are identical. This
    occurs as the logical second operand of "&&".

Change-Id: I87529e6a3801358bafe265869125c2eff2d40587
Reviewed-on: http://review.couchbase.org/70168
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>