Fix incorrect refantenna check

Created by: aroffringa

Possible fix for #212 (closed) , crash related to TECConstraint. Bug was introduced in #191 . Bug should only lead to a crash when all antennas are >20% flagged, which is inconsistent with what Francesco reports in https://github.com/lofar-astron/DP3/issues/212#issuecomment-583316960 . Nevertheless, this commit fixes a bug, and I don't see any other issues in the refant calculation.

This bug was btw already more or less mentioned by me in my review of #191, apparently the fix didn't make it properly and it got merged unnoticed :( .