TsCodeGenerator crashes when testing empty namespace

[coder7695]

The generateEntry method of idl_gen_ts creates new NameSpace() when handling schema with empty namespace but does not delete the NameSpace object on reuse and causes memory leak. Log attached:

=================================================================
==37831==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 32 byte(s) in 1 object(s) allocated from:
    #0 0x5a7eb38a7461 in operator new(unsigned long) (~/flatbuffers/tests/fuzzer/codegen_fuzzer+0x11ce461) (BuildId: b735958c5495c0ec)
    #1 0x5a7eb40579dd in flatbuffers::ts::TsGenerator::generateEntry() ~/flatbuffers/src/idl_gen_ts.cpp:263:18
    #2 0x5a7eb404812d in flatbuffers::ts::TsGenerator::generate() ~/flatbuffers/src/idl_gen_ts.cpp:120:7
    #3 0x5a7eb4046906 in flatbuffers::GenerateTS(flatbuffers::Parser const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&) ~/flatbuffers/src/idl_gen_ts.cpp:2286:20
    #4 0x5a7eb40453c1 in flatbuffers::(anonymous namespace)::TsCodeGenerator::GenerateCode(flatbuffers::Parser const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&) ~/flatbuffers/src/idl_gen_ts.cpp:2310:10
    #5 0x5a7eb38adb6f in LLVMFuzzerTestOneInput ~/flatbuffers/tests/fuzzer/flatbuffers_codegen_fuzzer.cc:139:24
    #6 0x5a7eb37b4c74 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) crtstuff.c
    #7 0x5a7eb37b4369 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) crtstuff.c
    #8 0x5a7eb37b5b55 in fuzzer::Fuzzer::MutateAndTestOne() crtstuff.c
    #9 0x5a7eb37b66b5 in fuzzer::Fuzzer::Loop(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) crtstuff.c
    #10 0x5a7eb37a396f in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) crtstuff.c
    #11 0x5a7eb37ce056 in main (~/flatbuffers/tests/fuzzer/codegen_fuzzer+0x10f5056) (BuildId: b735958c5495c0ec)
    #12 0x70199002a1c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #13 0x70199002a28a in __libc_start_main csu/../csu/libc-start.c:360:3
    #14 0x5a7eb3798954 in _start (~/flatbuffers/tests/fuzzer/codegen_fuzzer+0x10bf954) (BuildId: b735958c5495c0ec)

SUMMARY: AddressSanitizer: 32 byte(s) leaked in 1 allocation(s).
INFO: to ignore leaks on libFuzzer side use -detect_leaks=0.

MS: 1 CopyPart-; base unit: f63a738895d4d93bdd7f04ac25408a8eb7f40546
0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x61,0x74,0x74,0x72,0x69,0x62,0x75,0x74,0x65,0x20,0x74,0x61,0x62,0x6c,0x65,0x3b,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0xa,0x20,0x20,0x20,0x20,0x20,0x20,0x20,0x20,
                      attribute table;                   \012
artifact_prefix='./'; Test unit written to ./leak-605e2fd48a5fc7baa866130a8f605463508c534c
Base64: ICAgICAgICAgICAgICAgICAgICAgIGF0dHJpYnV0ZSB0YWJsZTsgICAgICAgICAgICAgICAgICAgCiAgICAgICAg