BUG: merge_asof raises when grouping on multiple columns with a categorical
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> x = pd.DataFrame(dict(x=[0],y=[0],z=pd.Categorical([0])))
>>> pd.merge_asof(x, x, on='x', by=['y', 'z'])

Traceback (most recent call last):
  File "bug.py", line 10, in <module>
    pd.merge_asof(x, x, on='x', by=['y', 'z'])
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 486, in merge_asof
    return op.get_result()
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 1019, in get_result
    join_index, left_indexer, right_indexer = self._get_join_info()
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 734, in _get_join_info
    right_indexer) = self._get_join_indexers()
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 1269, in _get_join_indexers
    left_by_values = flip(left_by_values)
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 1231, in flip
    return np.array(lzip(*xs), labeled_dtypes)
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/dtypes/dtypes.py", line 62, in __repr__
    return str(self)
  File "~/anaconda3/envs/pantheon/lib/python3.6/site-packages/pandas/core/dtypes/dtypes.py", line 41, in __str__
    return self.__unicode__()
SystemError: PyEval_EvalFrameEx returned a result with an error set

```
#### Problem description

`merge_asof` takes a `by` argument which defines the groups to merge between. When `by` is any single column, or multiple non-categorical columns, the merge succeeds. When `by` includes multiple columns, at least one of which is categorical, an error is raised.

#### Expected Output

```python
   x  y  z
0  0  0  0
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.10-100.fc24.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C
LOCALE: None.None

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 4.2.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0
tables: None
numexpr: 2.6.2
feather: None
matplotlib: 2.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
