E.g. in case of a disconnected thermistor, or when it is really
really cold. Make sure our parsing doesn't get confuse by this and
thinks that
ok T:150.0/210.0 T0:-55.7/0 T1:150.0/210.0
is actually only T and T1 being reported by the firmware, causing
stuff to be wrongly canonicalized thanks to T/T1 being Smoothie's way
of reporting dual extruder setups.
Fixes#2007