avoid using &vector[0], prefer vector.data()

Fixes #310 (closed). This should not invoke undefined behavior for empty vectors

Event/EventPacker/src/lib/PackedDataBuffer.cpp

@@ -24,8 +24,8 @@ namespace LHCb::Hlt::PackedData {
     output.clear();
     output.resize( buffer_size ); // TODO get rid of unneeded initialization
 
-    zipMultipleAlgorithm( level, &input_size, (char*)&( m_buffer[0] ), &buffer_size, (char*)&( output[0] ),
-                          &output_size, compression );
+    zipMultipleAlgorithm( level, &input_size, (char*)m_buffer.data(), &buffer_size, (char*)output.data(), &output_size,
+                          compression );
 
     if ( output_size == 0 || output_size > input_size ) {
       // the input cannot be compressed or size would be greater than original