Trying to preheat a non-existent extruder causes a crash.

As noted by @snowzach in #3461

Thanks to @gans-AD – #3363

Also removes the re-definition of now inside M190's while loop

Sorry for not having realized someone (me) redefined not existing servo pinns to -1, some lines above.
This is a simple error - not a feature.

SERVO2 is doubled defined and causes compiler errors

Since residency_start_ms is -1 when entering the heatup while loop whilst the hotend temp is close to the target them already then it may not escape.

Hence "Start the TEMP_RESIDENCY_TIME timer when we reach target temp for the first time."

This may fix a subtle bug caused by doing `G29` more than once without
`G28` between.