MB-36314: Seg Fault in scan operation when magma NewSeqInterator fails
MB-36314: Seg Fault in scan operation when magma NewSeqInterator fails

When magma::NewSeqIterator fails for any reason, it returns a null
iterator. The magma wrapper code was not checking for this condition.

Change-Id: I5b7c960c94e27d4d70dea3d69a5b712ca9c4e4fc
Reviewed-on: http://review.couchbase.org/115880
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@couchbase.com>
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine