Fix crashing bug in tokenizer, when tokenizing files with non-ASCII bytes

Fix crashing bug in tokenizer, when tokenizing files with non-ASCII bytes
but without a specified encoding: decoding_fgets() (and decoding_feof()) can
return NULL and fiddle with the 'tok' struct, making tok->buf NULL. This is
okay in the other cases of calls to decoding_*(), it seems, but not in this
one.

This should get a test added, somewhere, but the testsuite doesn't seem to
test encoding anywhere (although plenty of tests use it.)

It seems to me that decoding errors in other places in the code (like at the
start of a token, instead of in the middle of one) make the code end up
adding small integers to NULL pointers, but happen to check for error states
before using the calculated new pointers. I haven't been able to trigger any
other crashes, in any case.

I would nominate this file for a comlete rewrite for Py3k. The whole
decoding trick is too bolted-on for my tastes.