Problem: Compilation error due to comparison between signed and unsigned expressions

Solution: Cast the signed expression (which is always positive) to unsigned
Signed-off-by: Guido Vranken <guidovranken@gmail.com>

src/v2_decoder.cpp

@@ -115,7 +115,8 @@ int zmq::v2_decoder_t::size_ready (uint64_t msg_size_,
 
     shared_message_memory_allocator &allocator = get_allocator ();
     if (unlikely (!_zero_copy
-                  || msg_size_ > allocator.data () + allocator.size () - read_pos_ )) {
+                  || msg_size_ >
+                  (size_t)(allocator.data () + allocator.size () - read_pos_))) {
         // a new message has started, but the size would exceed the pre-allocated arena
         // this happens every time when a message does not fit completely into the buffer
         rc = _in_progress.init_size (static_cast<size_t> (msg_size_));