Web-MQTT: don't call FHC when connection terminates early

[gomoripeti]

Proposed Changes

When the Web-MQTT connection terminates early because of no supported subprotocol, terminate/3 called fhc release although no fhc obtain was called yet. This was the case even when use_file_handle_cache was false, because #state.should_use_fhc was not initialized.

Fixing this avoids the below harmless warning

[debug] error updating ets counter <0.1224.0> in table #Ref<0.2797411137.1366163457.189876>:
[{ets, update_counter,
  [#Ref<0.2797411137.1366163457.189876>,
   <0.1224.0>, {5, -1}],
   ...
[warning] FHC: failed to update counter 'obtained_socket', client pid: <0.1224.0>

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[gomoripeti]

looks like many test suites failed which are not related to Web-MQTT

[johanrhodin]

An easy way to reproduce this issue is:

• install wscat https://github.com/websockets/wscat
• wscat -c ws://localhost:15675/ws/