Fix JVM segfault during JNI call. Part of [#2647]

Motivation:
Currently when Native.writev(...) is used it is possible to see a JVM segfault because the offset is updated to early.

Modification:
Only update the offset once it is safe to do so.

Result:
No more segfault

transport-native-epoll/src/main/c/io_netty_channel_epoll_Native.c

@@ -761,8 +761,6 @@ JNIEXPORT jlong JNICALL Java_io_netty_channel_epoll_Native_writev(JNIEnv * env,
         }
 
         w += res;
-        offset += loop;
-        length -= loop;
 
         // update the position of the written buffers
         int written = res;
@@ -791,6 +789,8 @@ JNIEXPORT jlong JNICALL Java_io_netty_channel_epoll_Native_writev(JNIEnv * env,
             }
 
         }
+        offset += loop;
+        length -= loop;
     }
     return w;
 }