clang UBSAN and sequence_stack

[alextibbles]

With clang-10 default set of UBSAN checks, the following gets triggered: /usr/include/boost/xpressive/detail/utility/sequence_stack.hpp:217:21: runtime error: applying non-zero offset 4 to null pointer

#include <boost/xpressive/detail/utility/sequence_stack.hpp>

int main() {
    boost::xpressive::detail::sequence_stack<int> s;
    s.push_sequence(1, 42);
    return 0;
}

The following patch avoids the sanitizer error:

216,220c216,217
<         // Advance the high-water mark
<         this->curr_ += count;
< 
<         // Check to see if we have overflowed this buffer
<         if(std::less<void*>()(this->end_, this->curr_))
---
>         // Check to see if we would overflow this buffer
>         if((nullptr == ptr) || (std::less<void*>()(this->end_, ptr + count)))
222,224d218
<             // oops, back this out.
<             this->curr_ = ptr;
< 
227a222,224
>         
>         // Advance the high-water mark
>         this->curr_ += count;

Apologies, I couldn't find where the CI output for boost.org is published to see if this shows up in existing tests. Reproducer attached: sequence_stack_ubsan.zip

[alextibbles]

Looks like this is already fixed in develop.