Zero out httpd_state before deallocating
Zero out httpd_state before deallocating

When a connection is aborted by the HTTP server while it's still being processed it is possible to hit a null pointer dereference issue by jumping back to a protothread (outputpt) after its httpd_state has been freed. This can be triggered by sending a POST to any form in the CC26xx web demo server using Firefox.
This patch prevents that by zeroing out httpd_state structs before freeing them, thus also clearing the httpd_state->outputpt field.
Tested using Firefox 55.0a1 on a CC2650 LaunchPad.

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