RuntimeWarning: invalid value encountered in power

[certik]

See here: http://travis-ci.org/#!/numpy/numpy/jobs/2337235

======================================================================
2784ERROR: Test a special case for var
2785----------------------------------------------------------------------
2786Traceback (most recent call last):
2787  File "/home/travis/virtualenv/python2.7/local/lib/python2.7/site-packages/numpy/ma/tests/test_core.py", line 2731, in test_varstd_specialcases
2788    _ = method(out=mout)
2789  File "/home/travis/virtualenv/python2.7/local/lib/python2.7/site-packages/numpy/ma/core.py", line 4780, in std
2790    np.power(out, 0.5, out=out, casting='unsafe')
2791RuntimeWarning: invalid value encountered in power

This is related to #394.

[certik]

This was fixed by ea23de8.