clang-9 on x86/64 has reported following warnings/errors:

 libubus-acl.c:123:2: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 libubus-io.c:108:18: error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
 libubus-io.c:395:56: error: comparison of integers of different signs: 'ssize_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
 libubus-req.c:441:4: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:119:18: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd_acl.c:152:5: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:348:3: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:352:3: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:357:3: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:362:3: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:367:3: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
 ubusd_acl.c:447:16: error: comparison of integers of different signs: 'int' and '__size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
 ubusd_acl.c:502:18: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd.c:123:13: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd.c:170:15: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd.c:262:43: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd.c:287:30: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd_event.c:170:18: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
 ubusd_obj.c:71:2: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]

Signed-off-by: Petr Štetiar <ynezz@true.cz>