Fix double destructor call in channel iterators

[a2Fsa2k]

Summary

• Fixes a double destructor call in buffered_channel and unbuffered_channel iterators that causes undefined behavior (double-free) with non-trivially-destructible types.
• Removes a redundant destructor call in operator++() that was already handled inside increment_().
• Adds regression tests using a type with an atomic destruction counter.

Problem

When iterating over a buffered_channel<T> or unbuffered_channel<T> with range-for where T has a non-trivial destructor (e.g., std::string, shared_ptr), the destructor is called twice on every increment. This is a double-free — undefined behavior that manifests as crashes or corruption.

Root cause

Two overlapping commits created the double-destroy:

• Commit 37b5f77 (Resource leak in channels using iterators #209) added ~value_type() in operator++()
• Commit e440623 (buffered_channel with range-for syntax leaks #258) later added the same ~value_type() inside increment_() guarded by !initial

Since operator++() calls increment_(), the destructor runs twice on the same dead object.

Fix

Remove the explicit destructor call in operator++() from both buffered_channel.hpp:412 and unbuffered_channel.hpp:447. The call in increment_() handles it correctly.

Testing

• Existing tests pass (they used int, which masked the bug — trivially destructible).
• New test_rangefor_non_trivial_dtor tests in both channel test files use counted_type, a type with an atomic destruction counter, verifying each object is destroyed exactly once.
• Both headers compile clean with -fsyntax-only.

Files changed

File	Lines	Change
include/boost/fiber/buffered_channel.hpp	-1	Remove redundant ~value_type()
include/boost/fiber/unbuffered_channel.hpp	-1	Remove redundant ~value_type()
test/test_buffered_channel_post.cpp	+75	Regression test
test/test_unbuffered_channel_post.cpp	+75	Regression test