Merge pull request #152 from loganb/Marlin_v1

Fix compile error on boards without a bed heater.

I could not find why this problem was introduced. 
Its quite obvious, so I think its in general interest to merge.

Marlin/temperature.cpp

@@ -725,7 +725,9 @@ void min_temp_error(uint8_t e) {
 }
 
 void bed_max_temp_error(void) {
+#if HEATER_BED_PIN > -1
   WRITE(HEATER_BED_PIN, 0);
+#endif
   if(IsStopped() == false) {
     SERIAL_ERROR_START;
     SERIAL_ERRORLNPGM("Temperature heated bed switched off. MAXTEMP triggered !!");