Fix unreachable code warning in buffers_cat.hpp

[ashtum]

It seems we can't simply assume that the code for past_end::operator*() will never compile, as the following code clearly needs to handle that case:

return mp11::mp_with_index<
    sizeof...(Bn) + 2>(
        it_.index(), // Considering the index is a runtime value, the compiler needs to generate code for when it points to `past_end`
        dereference{*this});

Fixes #2678

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6adca47) 93.01% compared to head (9c773e7) 93.04%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2803      +/-   ##
===========================================
+ Coverage    93.01%   93.04%   +0.02%     
===========================================
  Files          178      178              
  Lines        13688    13688              
===========================================
+ Hits         12732    12736       +4     
+ Misses         956      952       -4     

Files	Coverage Δ
include/boost/beast/core/impl/buffers_cat.hpp	98.42% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[vinniefalco]

Interesting, is this why we keep getting that damn warning !?

[ashtum]

Interesting, is this why we keep getting that damn warning !?

I believe so. The reason we are getting those warnings is that the returned net::mutable_buffer from the past_end::operator* function (which is marked unreachable) implicitly converts to net::const_buffer. For example, the following generates the warning:
https://godbolt.org/z/83G5f4Wx7

auto buf = buffers_cat(net::const_buffer{}, net::const_buffer{});
auto it  = buf.begin();
*it;

However, this doesn't:
https://godbolt.org/z/WePad646W

auto buf = buffers_cat(net::mutable_buffer{}, net::mutable_buffer{});
auto it  = buf.begin();
*it;

This is quite comparable to this example:
https://godbolt.org/z/8fMroWh54

[[noreturn]] const char * f1()
{
  __assume(false);
}

const char * f2()
{
  return f1(); // Fine
}

std::string f3()
{
  return f1(); // warning C4702: unreachable code
}

MSVC is correctly warning us that we are doing something after unreachable code (in this case, the implicit conversion is what we are doing).