fix for the mapped_file_test compilation error for msvc 7.1

[David Walthall <walthall@…>]

he following patch fixes the mapped_file_test compilation error for msvc 7.1 (daw-msvc71):

Index: libs/iostreams/src/mapped_file.cpp =================================================================== --- libs/iostreams/src/mapped_file.cpp (revision 48407) +++ libs/iostreams/src/mapped_file.cpp (working copy) @@ -434,7 +434,7 @@

void mapped_file_source::close() { pimpl_->close(); }

-mapped_file_source::operator safe_bool() const +mapped_file_source::operator mapped_file_source::safe_bool() const

{ return pimpl_->error() ? &safe_bool_helper::x : 0; }

bool mapped_file_source::operator!() const

[Vladimir Prus]

(In [53428]) Workaround msvc compiler error on mapped_file.cpp

Fixes #2278. Patch from David Walthall.