Fix automatic stepper assignment for X/Y/Z dual stepper drivers

Marlin/pins.h

 #ifndef Z_MS2_PIN
   #define Z_MS2_PIN -1
 #endif
+#ifndef E0_STEP_PIN
+  #define E0_STEP_PIN -1
+#endif
+#ifndef E0_DIR_PIN
+  #define E0_DIR_PIN -1
+#endif
+#ifndef E0_ENABLE_PIN
+  #define E0_ENABLE_PIN -1
+#endif
 #ifndef E0_MS1_PIN
   #define E0_MS1_PIN -1
 #endif
 #ifndef E0_MS2_PIN
   #define E0_MS2_PIN -1
 #endif
+#ifndef E1_STEP_PIN
+  #define E1_STEP_PIN -1
+#endif
+#ifndef E1_DIR_PIN
+  #define E1_DIR_PIN -1
+#endif
+#ifndef E1_ENABLE_PIN
+  #define E1_ENABLE_PIN -1
+#endif
 #ifndef E1_MS1_PIN
   #define E1_MS1_PIN -1
 #endif
 #ifndef E1_MS2_PIN
   #define E1_MS2_PIN -1
 #endif
+#ifndef E2_STEP_PIN
+  #define E2_STEP_PIN -1
+#endif
+#ifndef E2_DIR_PIN
+  #define E2_DIR_PIN -1
+#endif
+#ifndef E2_ENABLE_PIN
+  #define E2_ENABLE_PIN -1
+#endif
+#ifndef E3_STEP_PIN
+  #define E3_STEP_PIN -1
+#endif
+#ifndef E3_DIR_PIN
+  #define E3_DIR_PIN -1
+#endif
+#ifndef E3_ENABLE_PIN
+  #define E3_ENABLE_PIN -1
+#endif
+#ifndef E4_STEP_PIN
+  #define E4_STEP_PIN -1
+#endif
+#ifndef E4_DIR_PIN
+  #define E4_DIR_PIN -1
+#endif
+#ifndef E4_ENABLE_PIN
+  #define E4_ENABLE_PIN -1
+#endif
 
 #ifndef FAN_PIN
   #define FAN_PIN -1
     #define X2_STEP_PIN   _EPIN(E_STEPPERS, STEP)
     #define X2_DIR_PIN    _EPIN(E_STEPPERS, DIR)
     #define X2_ENABLE_PIN _EPIN(E_STEPPERS, ENABLE)
-    #if X2_ENABLE_PIN == 0
+    #if E_STEPPERS > 4 || !PIN_EXISTS(X2_ENABLE)
       #error "No E stepper plug left for X2!"
     #endif
   #endif
     #define Y2_STEP_PIN   _EPIN(Y2_E_INDEX, STEP)
     #define Y2_DIR_PIN    _EPIN(Y2_E_INDEX, DIR)
     #define Y2_ENABLE_PIN _EPIN(Y2_E_INDEX, ENABLE)
-    #if Y2_ENABLE_PIN == 0
+    #if Y2_E_INDEX > 4 || !PIN_EXISTS(Y2_ENABLE)
       #error "No E stepper plug left for Y2!"
     #endif
   #endif
     #define Z2_STEP_PIN   _EPIN(Z2_E_INDEX, STEP)
     #define Z2_DIR_PIN    _EPIN(Z2_E_INDEX, DIR)
     #define Z2_ENABLE_PIN _EPIN(Z2_E_INDEX, ENABLE)
-    #if Z2_ENABLE_PIN == 0
+    #if Z2_E_INDEX > 4 || !PIN_EXISTS(Z2_ENABLE)
       #error "No E stepper plug left for Z2!"
     #endif
   #endif