Fix null-pointer free

Own error in recent decimeter commit

source/blender/bmesh/intern/bmesh_polygon.c

@@ -1036,6 +1036,7 @@ void BM_face_triangulate(
         const int quad_method,
         const int ngon_method,
         const bool use_tag,
+        /* use for ngons only! */
         MemArena *pf_arena,
 
         /* use for MOD_TRIANGULATE_NGON_BEAUTY only! */

source/blender/bmesh/tools/bmesh_decimate_collapse.c

@@ -603,9 +603,8 @@ static bool bm_decim_triangulate_begin(BMesh *bm, int *r_edges_tri_tot)
 			faces_double = next;
 		}
 
-		BLI_memarena_free(pf_arena);
-
 		if (has_ngon) {
+			BLI_memarena_free(pf_arena);
 			BLI_heap_free(pf_heap, NULL);
 			BLI_edgehash_free(pf_ehash, NULL);
 		}