Fix minor inconsistency in type cast in voip example

because m_samples is a vector of Int16 values.

examples/voip/Server.cpp

@@ -85,7 +85,7 @@ class NetworkAudioStream : public sf::SoundStream
         // (don't forget that we run in two separate threads)
         {
             sf::Lock lock(m_mutex);
-            m_tempBuffer.assign(m_samples.begin() + static_cast<std::vector<sf::Int64>::difference_type>(m_offset), m_samples.end());
+            m_tempBuffer.assign(m_samples.begin() + static_cast<std::vector<sf::Int16>::difference_type>(m_offset), m_samples.end());
         }
 
         // Fill audio data to pass to the stream