Bugfix: test number for zero before devision

This bug was noted when we compared two numbers of the order 1e-11
and used a maxiumun relative deviation of 1e-5

  TEST_CLOSE_REL(1.0e-11, 1.1e-11, 1e-5);

This test always succeeded because 1.0e-11 < 1e-5.
This behaviour is not correct.