Fix a documented memory leak.

Despite the old comments, re-initing the msg_t leaks a refcount to
metadata in some situations.

v1_decoder looks like it isn't tested any more, but it seems like a good
idea to fix it because it has the exact same piece of buggy code
v2_decoder does.

src/v1_decoder.cpp

@@ -80,10 +80,9 @@ int zmq::v1_decoder_t::one_byte_size_ready (unsigned char const*)
             return -1;
         }
 
-        //  in_progress is initialised at this point so in theory we should
-        //  close it before calling zmq_msg_init_size, however, it's a 0-byte
-        //  message and thus we can treat it as uninitialised...
-        int rc = in_progress.init_size (*tmpbuf - 1);
+        int rc = in_progress.close();
+        assert(rc == 0);
+        rc = in_progress.init_size (*tmpbuf - 1);
         if (rc != 0) {
             errno_assert (errno == ENOMEM);
             rc = in_progress.init ();
@@ -123,10 +122,9 @@ int zmq::v1_decoder_t::eight_byte_size_ready (unsigned char const*)
 
     const size_t msg_size = static_cast <size_t> (payload_length - 1);
 
-    //  in_progress is initialised at this point so in theory we should
-    //  close it before calling init_size, however, it's a 0-byte
-    //  message and thus we can treat it as uninitialised...
-    int rc = in_progress.init_size (msg_size);
+    int rc = in_progress.close();
+    assert(rc == 0);
+    rc = in_progress.init_size (msg_size);
     if (rc != 0) {
         errno_assert (errno == ENOMEM);
         rc = in_progress.init ();

src/v2_decoder.cpp

@@ -108,10 +108,8 @@ int zmq::v2_decoder_t::size_ready(uint64_t msg_size, unsigned char const* read_p
         return -1;
     }
 
-    //  in_progress is initialised at this point so in theory we should
-    //  close it before calling init_size, however, it's a 0-byte
-    //  message and thus we can treat it as uninitialised.
-    int rc = -1;
+    int rc = in_progress.close();
+    assert(rc == 0);
 
     // the current message can exceed the current buffer. We have to copy the buffer
     // data into a new message and complete it in the next receive.