Add depth/index to decode depth error message

Include depth and character index when throwing decode nesting errors.

Pre-emptively add a test decoding a massively nested JSON array. Lua
stack overflow faults are unlikely to occur on simple data structures.
Valgrind can highlight stack allocation bugs with complicated JSON even
if the test succeeds.

lua_cjson.c

@@ -1034,7 +1034,7 @@ static void json_next_number_token(json_parse_t *json, json_token_t *token)
  */
 static void json_next_token(json_parse_t *json, json_token_t *token)
 {
-    json_token_type_t *ch2token = json->cfg->ch2token;
+    const json_token_type_t *ch2token = json->cfg->ch2token;
     int ch;
 
     /* Eat whitespace. */
@@ -1149,7 +1149,8 @@ static void json_decode_descend(lua_State *l, json_parse_t *json, int slots)
     }
 
     strbuf_free(json->tmp);
-    luaL_error(l, "Too many nested data structures");
+    luaL_error(l, "Found too many nested data structures (%d) at character %d",
+        json->current_depth, json->ptr - json->data);
 }
 
 static void json_parse_object_context(lua_State *l, json_parse_t *json)

tests/test.lua

@@ -162,15 +162,18 @@ local cjson_tests = {
       true, { {{{{{ "nested" }}}}} } },
     { "Decode array over nested limit [throw error]",
       json.decode, { '[[[[[[ "nested" ]]]]]]' },
-      false, { "Too many nested data structures" } },
+      false, { "Found too many nested data structures (6) at character 6" } },
     { "Decode object at nested limit",
       json.decode, { '{"a":{"b":{"c":{"d":{"e":"nested"}}}}}' },
       true, { {a={b={c={d={e="nested"}}}}} } },
     { "Decode object over nested limit [throw error]",
       json.decode, { '{"a":{"b":{"c":{"d":{"e":{"f":"nested"}}}}}}' },
-      false, { "Too many nested data structures" } },
+      false, { "Found too many nested data structures (6) at character 26" } },
     { "Set decode_max_depth(1000)",
       json.decode_max_depth, { 1000 }, true, { 1000 } },
+    { "Decode deeply nested array [throw error]",
+      json.decode, { string.rep("[", 1100) .. '1100' .. string.rep("]", 1100)},
+      false, { "Found too many nested data structures (1001) at character 1001" } },
 
     -- Test encoding nested tables
     { "Set encode_max_depth(5)",