boot_serial: Initialize possibly initialized variable
boot_serial: Initialize possibly initialized variable

The variable `dec_off` can possibly be uninitialized in this function.
As long as the serial protocol used is formed correctly, the
initialization will always happen.  Place an initialization upon
function entry to protect against a case where a malformed protocol is
able to provoke this variable being used before being initialized.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot