Traceback when calling df.as_matrix()
I have a DataFrame object that was created by importing some data in list form, pivoting it, then adding totals columns using `df.append(pandas.DataFrame(some_matrix, columns=df.columns, index=[some_tuple])` and `df[some_other_tuple] = some_array` respectively. The resulting DataFrame object looks like this:

```
>>> df
             baz1                 baz2                 Totals
             spam1  spam2  spam3  spam1  spam2  spam3        
foo    bar                                                   
foo1   bar1  NaN    20     NaN    50     NaN    NaN    NaN   
       bar2  NaN    NaN    30     NaN    NaN    NaN    NaN   
foo2   bar2  NaN    40     NaN    NaN    NaN    NaN    NaN   
foo3   bar1  NaN    NaN    NaN    60     NaN    NaN    NaN   
Totals       NaN    NaN    NaN    NaN    NaN    NaN    NaN   
```

Calling `df.as_matrix()` fails with the below traceback:

```
>>> df.as_matrix()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/frame.py", line 671, in as_matrix
    self._consolidate_inplace()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/generic.py", line 267, in _consolidate_inplace
    self._data = self._data.consolidate()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 516, in consolidate
    new_blocks = _consolidate(self.blocks, self.items)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 938, in _consolidate
    new_block = _merge_blocks(list(group_blocks), items)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 951, in _merge_blocks
    do_integrity_check=True)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 211, in make_block
    do_integrity_check=do_integrity_check)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 34, in __init__
    self._check_integrity()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 40, in _check_integrity
    return (self.ref_locs[1:] > self.ref_locs[:-1]).all()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/pandas/core/internals.py", line 47, in ref_locs
    assert((indexer != -1).all())
AssertionError
```
