V4L/DVB (6326): tuner-core.c: fe_has_signal() can return uninitialized value

Initialize strength to zero.

Thanks to Adrian Bunk, who spotted this with the Coverity checker.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>