Fix a bug when unsub from a non-subscribed parent topic

[xiazhvera]

Issue #, if available:
awslabs/aws-crt-nodejs#405

Description of changes:
The issue happens when the application tries to unsubscribe from a non-subscribed parent topic. As an example:

Subscribe(test/topic/child)
Unsubscribe(test/topic)

As the "test/topic" is never subscribed, the topic tree would not delete the topic.
However, topic tree would treat the topic as "deleted" and tries to update the underlying topic filter, which hits the assertion.
Fixed it by validating if the topic is deleted before proceed to next step.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a760f42) 80.34% compared to head (da001a3) 80.34%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #297   +/-   ##
=======================================
  Coverage   80.34%   80.34%           
=======================================
  Files          18       18           
  Lines        7571     7571           
=======================================
  Hits         6083     6083           
  Misses       1488     1488           

Impacted Files	Coverage Δ
source/topic_tree.c	89.51% <100.00%> (ø)

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