ANDROID: selftests: fix incfs_test

Fix incfs test build error:

incfs_test.c:4441:19: error: argument 2 is null but the corresponding
                      size argument 3 value is 1 [-Werror=nonnull]
 4441 |         TESTEQUAL(read(fd, NULL, 1), -1);
      |                   ^

Bug: 211066171

Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Change-Id: I028d02aef9938a9abe6c529756b89d7cb07507f2