Problem: Several problems found by Coverity Static Analyzer

Solution: The Coverity Static Code Analyzer was used on libzmq code and found
many issues with uninitialized member variables, some redefinition of variables
hidding previous instances of same variable name and a couple of functions
where return values were not checked, even though all other occurrences were
checked (e.g. init_size() return).

src/null_mechanism.cpp

@@ -157,7 +157,7 @@ int zmq::null_mechanism_t::process_handshake_command (msg_t *msg_)
     }
 
     if (rc == 0) {
-        int rc = msg_->close ();
+        rc = msg_->close ();
         errno_assert (rc == 0);
         rc = msg_->init ();
         errno_assert (rc == 0);