Make disconnected thermistor show temp as absolute zero again

In a previous commit I changed the temperature calculation to give more
accurate readings at extreme values. This change meant that a
disconnected thermistor no longer shows as absolute zero. Coupled with
the change in thermistor beta value, this meant that a disconnected
extruder thermistor would show as -28.2C, which is above the usual error
threshold of -30C. This commit introduces a special case for a
disconnected thermistor and restores the reading to absolute zero for
that case.