Mustache: fix segmentation fault for unexpected closing tag

[tyler92]

This patch fixes a bug identified as a vulnerability by OSS-Fuzz: https://issues.oss-fuzz.com/issues/42536654

AddressSanitizer:DEADLYSIGNAL
=================================================================
==259589==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x55e78391e6bb bp 0x7ffc4358c4a0 sp 0x7ffc4358c160 T0)
==259589==The signal is caused by a READ memory access.
==259589==Hint: this fault was caused by a dereference of a high value address (see register values below).  Disassemble the provided pc to learn which register was used.
    #0 0x55e78391e6bb in crow::mustache::template_t::parse() Crow/include/crow/mustache.h:545:73
    #1 0x55e78391bccb in crow::mustache::template_t::template_t(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>) Crow/include/crow/mustache.h:163:17
    #2 0x55e783919bcb in crow::mustache::compile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&) Crow/include/crow/mustache.h:736:20
    #3 0x55e783919bcb in LLVMFuzzerTestOneInput Crow/tests/fuzz/template_fuzzer.cpp:23:21
    #4 0x55e783820d30 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (template_fuzzer+0x65d30)
    #5 0x55e78380ae82 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (template_fuzzer+0x4fe82)
    #6 0x55e7838108c6 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (template_fuzzer+0x558c6)
    #7 0x55e783839a32 in main (template_fuzzer+0x7ea32)
    #8 0x7f0dc62cad8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #9 0x7f0dc62cae3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #10 0x55e783805e64 in _start (template_fuzzer+0x4ae64)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV Crow/include/crow/mustache.h:545:73 in crow::mustache::template_t::parse()
==259589==ABORTING

Mustache file:

{{/tag}}

There are two possible solutions: fail with an error or ignore such unexpected tags. I implemented the first one as more strict. Please let me know if you disagree.

Also there are two changes in template_fuzzer:

1. Catch std::exception instead of invalid_template_exception because render_string can throw std::runtime_error
2. Return 0 instead of -1 even for errors - for fuzzing it's a common practice. "-1" means "this input is not interesting for the fuzzer" but it's not our case here