dcerpc: prevent integer underflow

in case a fragment has a length lesser than DCERPC_HDR_LEN

Fixes: 9daf852 ("dcerpc: tidy up code")

Ticket: 7548

rust/src/dcerpc/dcerpc.rs

@@ -938,7 +938,7 @@ impl DCERPCState {
 
         let fraglen = self.get_hdr_fraglen().unwrap_or(0);
 
-        if cur_i.len() < (fraglen - frag_bytes_consumed) as usize {
+        if (cur_i.len() + frag_bytes_consumed as usize) < fraglen as usize {
             SCLogDebug!("Possibly fragmented data, waiting for more..");
             return AppLayerResult::incomplete(parsed as u32, fraglen as u32 - parsed as u32);
         }