Fixes #171 ("deque::clear() uses undefined behaviour")

doc/container.qbk

@@ -1341,6 +1341,8 @@ use [*Boost.Container]? There are several reasons for that:
 [section:release_notes_boost_1_76_00 Boost 1.76 Release]
 
 * Added [[no-discard]] attribute in all containers to catch bugs related to unused return values.
+* Fixed bugs/issues:
+   * [@https://github.com/boostorg/container/issues/171    GitHub #171: ['"deque::clear() uses undefined behaviour"]].
 
 [endsect]
 

include/boost/container/deque.hpp

@@ -1843,6 +1843,8 @@ class deque : protected deque_base<typename real_allocator<T, Allocator>::type,
    //! <b>Complexity</b>: Linear to the number of elements in the deque.
    void clear() BOOST_NOEXCEPT_OR_NOTHROW
    {
+      if (this->members_.m_finish != this->members_.m_start) {
+  
          for (index_pointer node = this->members_.m_start.m_node + 1;
                node < this->members_.m_finish.m_node;
                ++node) {
@@ -1860,6 +1862,7 @@ class deque : protected deque_base<typename real_allocator<T, Allocator>::type,
 
          this->members_.m_finish = this->members_.m_start;
       }
+   }
 
    //! <b>Effects</b>: Returns true if x and y are equal
    //!