Improve PDF repair logic.

When we meet a broken PDF file, we attempt to repair it. We do this by
reading tokens from the file and attempting to interpret them as a
normal PDF stream.

Unfortunately, if the file is corrupt enough so that we start to read
from the middle of a stream, and we happen to hit an '(' character,
we can go into string reading mode. We can then end up skipping over
vast swathes of file that we could otherwise repair.

We fix this here by using a new version of the pdf_lex function that
refuses to ever return a string. This means we may take more time
over skipping things than we did before, but are less likely to
skip stuff.

We also tweak other parts of the pdf repair logic here. If we hit a
badly formed piece of data, clear the num/gen we have stored so that
the next plausible piece we get does not get assigned to a random
object number.

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