sndhdr.what() function throws several types of exceptions on unknown files instead of returning None (as documentation says).

Following code can replicate these crashes:

```
import sndhdr
import sys

sndhdr.what(sys.argv[1])
```

First crash is from wave or chunk module (input data is base64 encoded in the echo command):

```
$ echo UklGRjAwMDBXQVZFZm10IDAwMDABADAwMDAwMDAwMDAwMDAw | python3.7 -mbase64 -d > in.file
$ python3.7 sndhdr/test.py in.file
Traceback (most recent call last):
  File "sndhdr/test.py", line 4, in <module>
    sndhdr.what(sys.argv[1])
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 54, in what
    res = whathdr(filename)
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 63, in whathdr
    res = tf(h, f)
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 163, in test_wav
    w = wave.open(f, 'r')
  File "/tmp/python-3.7-bin/lib/python3.7/wave.py", line 510, in open
    return Wave_read(f)
  File "/tmp/python-3.7-bin/lib/python3.7/wave.py", line 164, in __init__
    self.initfp(f)
  File "/tmp/python-3.7-bin/lib/python3.7/wave.py", line 153, in initfp
    chunk.skip()
  File "/tmp/python-3.7-bin/lib/python3.7/chunk.py", line 160, in skip
    self.file.seek(n, 1)
  File "/tmp/python-3.7-bin/lib/python3.7/chunk.py", line 113, in seek
    raise RuntimeError
RuntimeError
```

Second crash comes from sndhdr module itself (again base64 encoded data is first decoded on command line):

```
$ echo AAA= | python3.7 -mbase64 -d > in.file
$ python3.7 sndhdr/test.py in.fileTraceback (most recent call last):
  File "sndhdr/test.py", line 4, in <module>
    sndhdr.what(sys.argv[1])
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 54, in what
    res = whathdr(filename)
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 63, in whathdr
    res = tf(h, f)
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 192, in test_sndr
    rate = get_short_le(h[2:4])
  File "/tmp/python-3.7-bin/lib/python3.7/sndhdr.py", line 213, in get_short_le
    return (b[1] << 8) | b[0]
IndexError: index out of range
```