Coap fixes (#1606)

* Fixes segmentation fault in POSIX version by removing coap message initialisation with NULL header

* Alters API so that there will be no attempt to create messages with NULL header

src/cli/cli_coap.cpp

@@ -260,7 +260,7 @@ exit:
 ThreadError Coap::ProcessClient(int argc, char *argv[])
 {
     ThreadError error = kThreadError_None;
-    otMessage *message = otCoapNewMessage(sInstance, NULL);
+    otMessage *message = NULL;
     otMessageInfo messageInfo;
     otCoapHeader header;