rpc, internal: fix CodeQL int conversions warnings

```
Incorrect conversion of an integer with architecture-dependent bit
size from to a lower bit size type int32 without an upper bound
check.
```

pkg/rpc/server/util.go

@@ -0,0 +1,20 @@
+package server
+
+import (
+	"errors"
+	"math"
+)
+
+func checkUint32(i int) error {
+	if i < 0 || i > math.MaxUint32 {
+		return errors.New("value should fit uint32")
+	}
+	return nil
+}
+
+func checkInt32(i int) error {
+	if i < math.MinInt32 || i > math.MaxInt32 {
+		return errors.New("value should fit int32")
+	}
+	return nil
+}