Fix the fuzzer by setting limits for depth and ext.

This is a short lived bug in the fuzzer implementation in which the
limits were not set correctly.

Credit to OSS-Fuzz

fuzz/unpack_pack_fuzzer.cpp

@@ -10,6 +10,8 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
                                                       nullptr,
                                                       nullptr,
                                                       msgpack::unpack_limit(test_limit,
+                                                                            test_limit,
+                                                                            test_limit,
                                                                             test_limit,
                                                                             test_limit,
                                                                             test_limit));