Remove compiler warning in logging.cc

[jonricha]

There is an unsigned vs signed comparison warning in logging.cc. This should remove it. It is already guaranteed to be a positive number from the condition on line 53 where c is already known to be between '0' and '9', so c will be in the range of 0 to 9.

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[jonricha]

I signed it

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CLAs look good, thanks!

[maflcko]

Can be closed, since it is included in #492 and needs rebase.

[pwnall]

@jonricha Thank you for your contribution!

I didn't merge it because the old implementation of ConsumeDecimalNumber crashed inexplicably on ARM until we changed the type of a local variable, so I was afraid that a similar type change might bring the crashes back. I ended up reimplementing ConsumeDecimalNumber and this PR doesn't make sense in the context of the new implementation.

Also, sorry for the late response.