dnp3: fix signed integer overflow

By using unsigned integers everywhere

scripts/dnp3-gen/dnp3-gen.py

@@ -354,15 +354,15 @@ static int DNP3DecodeObjectG{{object.group}}V{{object.variation}}(const uint8_t
     DNP3PointList *points)
 {
     DNP3ObjectG{{object.group}}V{{object.variation}} *object = NULL;
-    int bytes = (count / 8) + 1;
+    uint32_t bytes = (count / 8) + 1;
     uint32_t prefix = 0;
-    int point_index = start;
+    uint32_t point_index = start;
 
     if (!DNP3ReadPrefix(buf, len, prefix_code, &prefix)) {
         goto error;
     }
 
-    for (int i = 0; i < bytes; i++) {
+    for (uint32_t i = 0; i < bytes; i++) {
 
         uint8_t octet;