qbsp: fix iterator debug assertion in ChopBrushes

"cannot decrement begin list iterator"

qbsp/brushbsp.cc

@@ -1475,9 +1475,9 @@ newlist:
 
 			if (c1 < c2) {
                 stats.c_from_split += sub.size();
-                auto before = std::prev(list.erase(b1_it)); // remove the current brush, go back one
-                list.splice(std::next(before), sub); // splice new list in place of where the brush was
-                b1_it = std::next(before); // restart list with the new brushes
+                auto before = list.erase(b1_it); // remove the current brush, go back one
+                list.splice(before, sub); // splice new list in place of where the brush was
+                b1_it = before; // restart list with the new brushes
 				goto newlist;
 			} else {
                 stats.c_from_split += sub2.size();

tests/test_qbsp.cc

@@ -1545,6 +1545,15 @@ TEST_CASE("quake maps", "[testmaps_q1][.releaseonly]")
     }
 }
 
+TEST_CASE("chop", "[testmaps_q1]")
+{
+    const auto [bsp, bspx, prt] = LoadTestmapQ1("quake_map_source/DM1-test.map", {"-chop", "-debugchop"});
+
+    CHECK(GAME_QUAKE == bsp.loadversion->game->id);
+    CHECK(prt);
+    CheckFilled(bsp);
+}
+
 TEST_CASE("base1leak", "[testmaps_q2]")
 {
     const auto [bsp, bspx, prt] = LoadTestmapQ2("base1leak.map");