FIX: fixed isOleFile issue when passing small buffer.

[Nazarii]

Fixes # .
Fixed file() argument 1 must be encoded string without NULL bytes, not str
Changes proposed in this pull request:

When calling isOleFile('Hello there') we will receive an error as method determines argument as a file but should return False instead of raising an error.
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…AL_OLEFILE_SIZE

[hugovk]

@Nazarii This fails an existing test on Python 2.x:

TestOleFileIo.test_isOleFile ... FAIL
======================================================================
FAIL: TestOleFileIo.test_isOleFile
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/python-pillow/Pillow/Tests/test_olefileio.py", line 13, in test_isOleFile
    self.assertTrue(OleFileIO.isOleFile(ole_file))
AssertionError: False is not true

https://travis-ci.org/python-pillow/Pillow/jobs/178017768
https://travis-ci.org/python-pillow/Pillow/builds/178017763

This PR would also need a test which demonstrates the error, fails on current master and passes with your change.

However, we may be removing olefile from this repo, see #2199. Even if we don't, this would probably also have to go to the main repo, with tests: https://github.com/decalage2/olefile