Remove compile warning about assinging to itself

warning: explicitly assigning a variable of type 'couchstore_error_t' to itself [-Wself-assign]
    do { if((errcode = (errcode)) < 0) { goto cleanup; } } while (0);
             ~~~~~~~ ^  ~~~~~~~

Change-Id: I97fadb7a4fc5aa66d6c3bc66634937d4223046d1
Reviewed-on: http://review.couchbase.org/31021
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>