eina log - keep coverity happy with potential ptr comparison overflow

eina log - keep coverity happy with potential ptr comparison overflow

fixes CID 1361220

in theory yes end minus start could be insanely huge or end be very
high in memory thus causing an overflow. this would have to be in the
last few bytes of memory space, so it never going to happen. and the
input from the env var has to be sane anyway as its user controlled.

@fix