onewire_parser: Fix crash caused by off by one memory allocation

onewire_parser: Fix crash caused by off by one memory allocation

The entries in sensor list always start with either server address or
mount point, which is not counted as a sensor. If we want to count the
actual number of sensors, we obviously need to add one more to the
last index number, otherwise we get one too small number for sensors
and allocate too little of memory for the parser state.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>