Fixed wrong buffer end detection in v2_decoder.

zero-copy msg_t::init cannot be used when the message exceeds either
the buffer end or the last received byte. To detect this, the buffer
is now resized to the numnber of received bytes.

src/v2_decoder.cpp

@@ -43,7 +43,8 @@
 
 zmq::shared_message_memory_allocator::shared_message_memory_allocator(size_t bufsize_):
     buf(NULL),
-    bufsize( bufsize_ )
+    bufsize( 0 ),
+    maxsize( bufsize_ )
 {
 
 }
@@ -71,6 +72,7 @@ unsigned char* zmq::shared_message_memory_allocator::allocate()
         new(buf) atomic_counter_t(1);
     }
 
+    bufsize = maxsize;
     return buf + sizeof( zmq::atomic_counter_t);
 }
 
@@ -78,12 +80,15 @@ void zmq::shared_message_memory_allocator::deallocate()
 {
     free(buf);
     buf = NULL;
+    bufsize = 0;
 }
 
 unsigned char* zmq::shared_message_memory_allocator::release()
 {
     unsigned char* b = buf;
     buf = NULL;
+    bufsize = 0;
+
     return b;
 }