Occasional crash when changing WIA sources.

[dynarithmic]

          You are right I've tested with WIA drivers and I only get one image even if the scanner is duplex.

On the other side, playing with the sources (changing from one driver to another) sometimes on the DTWAIN_SysDestroy call I get the exception:

ntdll.dll!77207073()    Unknown
ntdll.dll![Frames below may be incorrect and/or missing, no symbols loaded for ntdll.dll]    Unknown
[External Code]   
[Inline Frame] dtwain32.dll!boost::dll::detail::shared_library_impl::unload() Line 120    C++
[Inline Frame] dtwain32.dll!boost::dll::shared_library::unload() Line 286    C++
dtwain32.dll!dynarithmic::CTL_TwainAppMgr::UnloadSourceManager() Line 314    C++
dtwain32.dll!dynarithmic::CTL_TwainAppMgr::Destroy() Line 113    C++
dtwain32.dll!DTWAIN_EndTwainSession() Line 1562    C++
dtwain32.dll!DTWAIN_SysDestroy() Line 1623    C++

Originally posted by @gerardcarbo in #46 (comment)