If a timer fires after onDestroy is called, the ext_proc filter may crash while
trying to access invalid encoder or decoder filter callbacks. The documentation
for StreamFilterBase::onDestroy() says, "Filters must not invoke either
encoder or decoder filter callbacks after having onDestroy() invoked."

Signed-off-by: Rick Stewart <ristewart@google.com>

Signed-off-by: Rick Stewart <ristewart@google.com>

Signed-off-by: Rick Stewart <ristewart@google.com>

…s_on_decode

Signed-off-by: Rick Stewart <ristewart@google.com>