boost::local_time::posix_time_zone_base<char>::M_func may trigger an assert on invalid inputs

We have an routine that parses date-time strings coming in over protocol. The code used is:

```
boost::posix_time::ptime stringToDateTime(const std::string& string) {
static std::locale locale(std::locale::classic(), new boost::local_time::local_time_input_facet("%Y-%m-%d %H:%M:%S%F%ZP"));
std::istringstream stream(string);
stream.imbue(locale);
boost::local_time::local_date_time result(boost::date_time::not_a_date_time);
stream >> result;
return result.utc_time();
}
```

This normally works fine, but when AFL++ managed to generate input with the following timestamp: `20111111111111111111F,M,117-07-13:20:00:00Z`, boost asserted with the this backtrace:

```
frame #0: 0x000000018e973720 libsystem_kernel.dylib`__pthread_kill + 8
frame #1: 0x000000018e9abf70 libsystem_pthread.dylib`pthread_kill + 288
frame #2: 0x000000018e8b8908 libsystem_c.dylib`abort + 128
frame #3: 0x000000018e8b7c1c libsystem_c.dylib`__assert_rtn + 284
* frame #4: 0x0000000100e3bbc8 checker`boost::token_iterator>, std::__1::__wrap_iter, std::__1::basic_string, std::__1::allocator>>::increment(this=0x000000016fdfcda8) at token_iterator.hpp:51:11
frame #5: 0x0000000100e3bb64 checker`void boost::iterator_core_access::increment>, std::__1::__wrap_iter, std::__1::basic_string, std::__1::allocator>>>(f=0x000000016fdfcda8) at iterator_facade.hpp:520:13
frame #6: 0x0000000100e3ba80 checker`boost::iterator_facade>, std::__1::__wrap_iter, std::__1::basic_string, std::__1::allocator>>, std::__1::basic_string, std::__1::allocator>, boost::forward_traversal_tag, std::__1::basic_string, std::__1::allocator> const&, long>::operator++(this=0x000000016fdfcda8) at iterator_facade.hpp:660:11
frame #7: 0x0000000100e388cc checker`boost::detail::postfix_increment_result>, std::__1::__wrap_iter, std::__1::basic_string, std::__1::allocator>>, std::__1::basic_string, std::__1::allocator>, std::__1::basic_string, std::__1::allocator> const&, boost::forward_traversal_tag>::type boost::operator++>, std::__1::__wrap_iter, std::__1::basic_string, std::__1::allocator>>, std::__1::basic_string, std::__1::allocator>, boost::forward_traversal_tag, std::__1::basic_string, std::__1::allocator> const&, long>(i=0x000000016fdfcda8, (null)=0) at iterator_facade.hpp:730:7
frame #8: 0x0000000100e3d2c0 checker`boost::local_time::posix_time_zone_base::M_func(this=0x00006000000a83c0, s="M", e="117-07-13:20:00:00Z") at posix_time_zone.hpp:393:42
frame #9: 0x0000000100e38aa4 checker`boost::local_time::posix_time_zone_base::calc_rules(this=0x00006000000a83c0, start="M", end="117-07-13:20:00:00Z") at posix_time_zone.hpp:332:9
frame #10: 0x0000000100e37ca4 checker`boost::local_time::posix_time_zone_base::posix_time_zone_base(this=0x00006000000a83c0, s="1F,M,117-07-13:20:00:00Z") at posix_time_zone.hpp:115:9
frame #11: 0x0000000100e31760 checker`boost::local_time::posix_time_zone_base::posix_time_zone_base(this=0x00006000000a83c0, s="1F,M,117-07-13:20:00:00Z") at posix_time_zone.hpp:92:5
frame #12: 0x0000000100e2d530 checker`std::__1::basic_istream>& boost::local_time::operator>>>(is=0x000000016fdfd9b0, ldt=0x000000016fdfd990) at local_time_io.hpp:91:36
```

It looks like the iterator increments in `boost::local_time::posix_time_zone_base::M_func` are performed blindly, which means the code hits the `BOOST_ASSERT` in the token iterator that detects increments of invalid iterators, instead of returning a `not_a_date_time`, or (if enabled with `stream::exceptions`) throwing an exception.