move error check to a better place

router/java/src/net/i2p/router/transport/udp/PacketBuilder.java

@@ -238,10 +238,7 @@ class PacketBuilder {
             packet.release();
             return null;
         }
-        if (dataSize == 0) {
-            // OK according to the protocol but if we send it, it's a bug
-            _log.error("Sending zero-size fragment " + fragment + " of " + state + " for " + peer);
-        }
+
         int currentMTU = peer.getMTU();
         int availableForAcks = currentMTU - MIN_DATA_PACKET_OVERHEAD - dataSize;
         int availableForExplicitAcks = availableForAcks;
@@ -382,6 +379,11 @@ class PacketBuilder {
         //    _log.debug("Size written: " + sizeWritten + " for fragment " + fragment 
         //               + " of " + state.getMessageId());
         }
+        // put this after writeFragment() since dataSize will be zero for use-after-free
+        if (dataSize == 0) {
+            // OK according to the protocol but if we send it, it's a bug
+            _log.error("Sending zero-size fragment " + fragment + " of " + state + " for " + peer);
+        }
         off += dataSize;