Remove comparisons that can never evaluate to true

Remove comparisons that can never evaluate to true

Using clang as the compiler results in 4 warnings like the following:

osrf_list.c:106:23: warning: comparison of unsigned expression < 0 is
always false [-Wtautological-compare]
        if(!list || position < 0) return NULL;
                    ~~~~~~~~ ^ ~

(Explanation: "position" is an unsigned int; thus the comparison to < 0
can never evaluate to true).

Signed-off-by: Dan Scott <dan@coffeecode.net>