Initialize slopes before the map loads.

src/p_setup.c

@@ -4274,6 +4274,8 @@ boolean P_LoadLevel(boolean fromnetsave, boolean reloadinggamestate)
 
 	P_MapStart(); // tmthing can be used starting from this point
 
+	P_InitSlopes();
+
 	if (!P_LoadMapFromFile())
 		return false;
 

src/p_slopes.c

@@ -635,9 +635,6 @@ pslope_t *P_SlopeById(UINT16 id)
 void P_SpawnSlopes(const boolean fromsave) {
 	size_t i;
 
-	slopelist = NULL;
-	slopecount = 0;
-
 	/// Generates vertex slopes.
 	SpawnVertexSlopes();
 
@@ -671,6 +668,13 @@ void P_SpawnSlopes(const boolean fromsave) {
 		}
 }
 
+/// Initializes slopes.
+void P_InitSlopes(void)
+{
+	slopelist = NULL;
+	slopecount = 0;
+}
+
 // ============================================================================
 //
 // Various utilities related to slopes

src/p_slopes.h

@@ -50,6 +50,7 @@ typedef enum
 void P_LinkSlopeThinkers (void);
 
 void P_CalculateSlopeNormal(pslope_t *slope);
+void P_InitSlopes(void);
 void P_SpawnSlopes(const boolean fromsave);
 
 //