[PATCH] os: prevent negative array index access (#80890)

[Julien Cristau]

On Tue, Jul  8, 2014 at 09:24:18 +1000, Peter Hutterer wrote:

> If an empty string is provided to LogMessageVerbSigSafe, the length of the
> printed string is 0.
> 
Do we know who calls this with an empty string?

Cheers,
Julien