When performing a comparison of a signed int and an unsigned int, the
signed int is first converted to an unsigned int, so that negative
values are being treated as big, positive values. This can become a
problem in an overread check, namely when an overread already happened.
So change the type of the variable containing the amount of bits that
need to be left to signed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>