linux: Fix memory corruption on stream stop
linux: Fix memory corruption on stream stop

The buffer (m_frameBuffer) must not be freed until the capture thread
(m_helperThread) is joined.

Fixes #41.

Signed-off-by: Christopher N. Hesse <c0e24b1b2a15a36c0edfd453d5b08b3c191243c8@gmail.com>

openpnp/openpnp-capture,trcwm/openpnp-capture,openpnp/openpnp-capture,openpnp/openpnp-capture,openpnp/openpnp-capture,trcwm/openpnp-capture,trcwm/openpnp-capture,openpnp/openpnp-capture,trcwm/openpnp-capture,trcwm/openpnp-capture,trcwm/openpnp-capture,openpnp/openpnp-capture