Initialize the verify_result field in mbedtls_ssl_session_free().
Previously we were just zeroising the entire session object, which would
yield a default 'success' value if the same object were reused.
Test that this initialisation is actually happening by setting
verify_result manually to zero and calling mbedtls_ssl_session_free() on
the session before checking its value.
Signed-off-by: David Horstmann <david.horstmann@arm.com>
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