lint: remove check for value less than zero on unsigned var

`incr` is of type `size_t` which is essentially a `unsigned int` type (according to ISO 1999 C it's an unsigned integer of at least 16 Bit). Since unsigned integers can't be smaller than zero the check is unnecessary (and will hopefully gets optimized away by the compiler anyways).
This PR removes the unnecessary check to silence `cppcheck`.
I guess the parameter type for incr is wrong. It should take a `ptrdiff_t`