Fix undefined behaviour on getting pointer to data in empty vector

`&vector[0]` is undefined if vector.empty(), causing build failure on MSVC

src/network/networkpacket.cpp

@@ -63,7 +63,7 @@ void NetworkPacket::putRawPacket(u8 *data, u32 datasize, session_t peer_id)
 
 	// split command and datas
 	m_command = readU16(&data[0]);
-	memcpy(&m_data[0], &data[2], m_datasize);
+	memcpy(m_data.data(), &data[2], m_datasize);
 }
 
 const char* NetworkPacket::getString(u32 from_offset)