Fixes and test cases for Bug#729758, Bug#729879

A fix and a test case for

https://bugs.launchpad.net/tarantool/+bug/729758
"SELECT fails with a disjunct and small LIMIT"

and

https://bugs.launchpad.net/tarantool/+bug/729879
"Zero limit is treated the same as no limit"

These were simple coding bugs in SELECT main loop.