Add validator for non negative integer

A value of 0 means overload protection is disabled.
A negative value doesn't really make sense.

Author: ansd

deps/rabbitmq_mqtt/priv/schema/rabbitmq_mqtt.schema

@@ -283,4 +283,4 @@ end}.
 %%
 %% {mailbox_soft_limit, 30},
 {mapping, "mqtt.mailbox_soft_limit", "rabbitmq_mqtt.mailbox_soft_limit",
-    [{datatype, integer}]}.
+    [{datatype, integer}, {validators, ["non_negative_integer"]}]}.