basic_stringbuf<>::str() deallocating external buffer

The program below aborts due to deallocating the external buffer extbuf.

test.cpp:
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#include <sstream> // for stringbuf
#include <memory> // for allocator

#include <cassert> // for assert()

char extbuf [3];

struct MyAlloc : std::allocator <char>
{
pointer allocate (size_type __n, std::allocator<void>::const_pointer = 0)

{ return new char [__n]; }

void deallocate (pointer __p, size_type)

{ assert (extbuf != __p); delete [] __p; }

};

int main ()
{
std::basic_stringbuf <char, std::char_traits <char>, MyAlloc> sbuf;

sbuf.pubsetbuf (extbuf, sizeof (extbuf));

const char* str = "abcdef";
sbuf.str (str);
assert (sbuf.str () == str);

return 0;
}
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The test output:
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test: test.cpp:16: void MyAlloc::deallocate(char*, unsigned int): Assertion `extbuf != __p' failed.
Aborted
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