Fix M107 when dual-nozzles share fan (#12819) (#12909)

Marlin/src/gcode/temperature/M106_M107.cpp

  * M107: Fan Off
  */
 void GcodeSuite::M107() {
-  thermalManager.set_fan_speed(parser.byteval('P', active_extruder), 0);
+  const uint8_t p = parser.byteval('P', MIN(active_extruder, FAN_COUNT - 1));
+  thermalManager.set_fan_speed(p, 0);
 }
 
 #endif // FAN_COUNT > 0