Fix possible C stack overflow with json.encode

src/ljson.hpp

@@ -76,7 +76,9 @@ static soup::UniquePtr<soup::JsonNode> checkJson(lua_State* L, int i)
 			while (lua_next(L, -2))
 			{
 				lua_pushvalue(L, -2);
+				luaE_incCstack(L);
 				arr->children.emplace_back(checkJson(L, -2));
+				L->nCcalls--;
 				lua_pop(L, 2);
 			}
 			lua_pop(L, 1);

testes/pluto/basic.pluto

@@ -1577,13 +1577,23 @@ do
     local function recursive_enc()
         local t = {}
         t.t = t
-        try
-            return require"json".encode(t)
-        catch e then
-            return e
+        return require"json".encode(t)
+    end
+    assert(select(2, pcall(recursive_enc)) == "C stack overflow")
+
+    local function overflow_enc()
+        local root = {}
+        do
+            local prev_t = root
+            for i = 1, 10000 do
+                local t = {}
+                prev_t:insert(t)
+                prev_t = t
+            end
         end
+        return require"json".encode(root)
     end
-    assert(recursive_enc() == "C stack overflow")
+    assert(select(2, pcall(overflow_enc)) == "C stack overflow")
 end
 do
     local xml = require "pluto:xml"