snmp_agent plugin: fix a few build warnings

snmp_agent plugin: fix a few build warnings

```
CC       src/snmp_agent_la-snmp_agent.lo
src/snmp_agent.c:166:25: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
      for (int i = 0; i < dd->oids_len; i++) {
                      ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:190:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < dd->oids_len; i++) {
                    ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:289:37: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  for (i = 0; i < key_len && offset < oid->oid_len; i++)
                             ~~~~~~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:355:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < dd->oids_len; i++)
                    ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:420:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < (*dd)->oids_len; i++)
                    ~ ^ ~~~~~~~~~~~~~~~
src/snmp_agent.c:429:25: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
      for (int i = 0; i < (*dd)->oids_len; i++)
                      ~ ^ ~~~~~~~~~~~~~~~
src/snmp_agent.c:440:25: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
      for (int i = 0; i < (*dd)->oids_len; i++)
                      ~ ^ ~~~~~~~~~~~~~~~
src/snmp_agent.c:532:20: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  assert(oid_index < values_num);
         ~~~~~~~~~ ^ ~~~~~~~~~~
/usr/include/assert.h:89:5: note: expanded from macro 'assert'
  ((expr)                                                               \
    ^~~~
src/snmp_agent.c:584:25: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
      for (int i = 0; i < dd->oids_len; i++) {
                      ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:768:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < dd->oids_len; i++) {
                    ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:806:25: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
      for (int i = 0; i < dd->oids_len; i++) {
                      ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:821:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < dd->oids_len; i++) {
                    ~ ^ ~~~~~~~~~~~~
src/snmp_agent.c:1287:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < dd->oids_len; i++) {
                    ~ ^ ~~~~~~~~~~~~
13 warnings generated.
```