Fix TestSBData.py test case (tested fix Linux/Mac)

- stop ignoring the error-codes in the 'error' variable
- removed out-of-bounds accesses with read-only array fields such as:

  self.assertTrue(data2.uint8[6] == 0, 'binary 0 terminator')

Since SBData wraps a (6-character) python string literal, trying to read the
null-terminator raises an exception. Instead, I replaced the out-of-bounds
read with a length-check.

Other out-of-bounds reads (via accessor function like SBData.GetUnsignedInt8)
don't throw and are OK. I just added asserts that errors are set for these
negative cases.

llvm-svn: 175223