garbled response file triggering segfault in evalresp

Trying to remove response with the following garbled instrument response information (problem in input/output units during filter stages) results in a segfault in evalresp:
```python
from obspy import read, read_inventory
inv = read_inventory("TM.SKLT..BHZ.xml", "STATIONXML")
st = read("TM.SKLT..BHZ")
st.attach_response(inv)
st.remove_response()
```
```
 EVRESP ERROR [File: <stdin>]):
	check_channel; units mismatch between stages,
	skipping to next response now
Segmentation fault
```

Files are available here: https://github.com/obspy/sandbox/tree/master/attachments/836