bpf: Add a check to make static analysers happy

bpf: Add a check to make static analysers happy

In [1] Dan Carpenter reported that the following code makes the
Smatch static analyser unhappy:

        17904       value = map->ops->map_lookup_elem(map, &i);
        17905       if (!value)
        17906               return -EINVAL;
    --> 17907       items[i - start] = value->xlated_off;

The analyser assumes that the `value` variable may contain an error
and thus it should be properly checked before the dereference.
On practice this will never happen as array maps do not return
error values in map_lookup_elem, but to make the Smatch and other
possible analysers happy this patch adds a formal check.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/aR2BN1Ix--8tmVrN@stanley.mountain/ [1]
Fixes: 493d9e0d6083 ("bpf, x86: add support for indirect jumps")
Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://lore.kernel.org/r/20251119112517.1091793-1-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>