BUG: Fixed bug in len(DataFrame.groupby) causing IndexError when there's a NaN-only column (issue11016)
This is the new PR for Fixed issue #11016 

generally we don't make _new_ PR's. just `push -f` the old ones. ok for now. but pls have a look at:

http://pandas.pydata.org/pandas-docs/stable/contributing.html
 @jreback Got it.  I will take a closer look at it. 
 So when I run `nosetests pandas/tests/test_groupby.py`, it gave me a lot of errors. This is on master branch from recent pydata/pandas. I think the errors are from recent commits rather than this PR. 
 you need to rebuild the c extensions
after u pull from master do this
 merged via https://github.com/pydata/pandas/commit/066783c6ea16115d2cbc75dca443e973e81ad754

thanks
 Thanks for fixing!
