util: Fix return value for virJSONValueFromString if it fails

Problem:
  "parser.head" is not NULL even if it's free'ed by "virJSONValueFree",
returning "parser.head" when "virJSONValueFromString" fails will cause
unexpected errors (libvirtd will crash sometimes), e.g.
  In function "qemuMonitorJSONArbitraryCommand":

        if (!(cmd = virJSONValueFromString(cmd_str)))
            goto cleanup;

        if (qemuMonitorJSONCommand(mon, cmd, &reply) < 0)
            goto cleanup;

        ......

     cleanup:
        virJSONValueFree(cmd);

  It will continues to send command to monitor even if "virJSONValueFromString"
is failed, and more worse, it trys to free "cmd" again.

  Crash example:
{"error":{"class":"QMPBadInputObject","desc":"Expected 'execute' in QMP input","data":{"expected":"execute"}}}
{"error":{"class":"QMPBadInputObject","desc":"Expected 'execute' in QMP input","data":{"expected":"execute"}}}
error: server closed connection:
error: unable to connect to '/var/run/libvirt/libvirt-sock', libvirtd may need to be started: Connection refused
error: failed to connect to the hypervisor

  This fix is to:
    1) return NULL for failure of "virJSONValueFromString",
    2) and it seems "virJSONValueFree" uses incorrect loop index for type
       of "VIR_JSON_TYPE_OBJECT", fix it together.

* src/util/json.c

src/util/json.c

@@ -65,7 +65,7 @@ void virJSONValueFree(virJSONValuePtr value)
 
     switch (value->type) {
     case VIR_JSON_TYPE_OBJECT:
-        for (i = 0 ; i < value->data.array.nvalues ; i++) {
+        for (i = 0 ; i < value->data.object.npairs; i++) {
             VIR_FREE(value->data.object.pairs[i].key);
             virJSONValueFree(value->data.object.pairs[i].value);
         }
@@ -897,6 +897,7 @@ virJSONValuePtr virJSONValueFromString(const char *jsonstring)
     yajl_parser_config cfg = { 1, 1 };
     yajl_handle hand;
     virJSONParser parser = { NULL, NULL, 0 };
+    virJSONValuePtr ret = NULL;
 
     VIR_DEBUG("string=%s", jsonstring);
 
@@ -917,6 +918,8 @@ virJSONValuePtr virJSONValueFromString(const char *jsonstring)
         goto cleanup;
     }
 
+    ret = parser.head;
+
 cleanup:
     yajl_free(hand);
 
@@ -930,7 +933,7 @@ cleanup:
 
     VIR_DEBUG("result=%p", parser.head);
 
-    return parser.head;
+    return ret;
 }