src/resource.c: Handle subscriber leak if app is not handling duplicates
src/resource.c: Handle subscriber leak if app is not handling duplicates

If multiple GET requests for the same URI + QUERY with different tokens and
Observe =0 is set, then for each GET there is an increment of the
subscriptions.

Currently, if the application is not clearing out the duplicate subscribers,
then the subscriber list gets large and eventually runs out of memory.

This fix stops duplicate subscriptions for the same session with the same
URI + QUERY, deleting the original entry and replacing it with a new
subscription.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap