apply: get rid of useless x < 0 comparison on a size_t type

apply: get rid of useless x < 0 comparison on a size_t type

According to the C standard size_t is always unsigned, therefore the
comparison "n1 < 0 || n2 < 0" when n1 and n2 are size_t will always be
false.

This was raised by clang 2.9 which throws this warning when compiling
apply.c:

    builtin/apply.c:253:9: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
            if (n1 < 0 || n2 < 0)
                ~~ ^ ~
    builtin/apply.c:253:19: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
            if (n1 < 0 || n2 < 0)
                          ~~ ^ ~

This check was originally added in v1.6.5-rc0~53^2 by Giuseppe Bilotta
while adding an option to git-apply to ignore whitespace differences.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>