Merge pull request #3421 from jbrazio/bugfix/m109-EXTRUDE_MINTEMP-expansion

M109: Protect against EXTRUDE_MINTEMP expansion

Marlin/Marlin_main.cpp

 
   // Prevents a wait-forever situation if R is misused i.e. M109 R0
   // Try to calculate a ballpark safe margin by halving EXTRUDE_MINTEMP
-  if (degTargetHotend(target_extruder) < (EXTRUDE_MINTEMP/2)) return;
+  if (degTargetHotend(target_extruder) < (EXTRUDE_MINTEMP)/2) return;
 
   #ifdef TEMP_RESIDENCY_TIME
     long residency_start_ms = -1;