fix wrong buffer size in path string conversion functions

Fixes boostorg#733.

include/boost/gil/io/path_spec.hpp

@@ -41,9 +41,11 @@ inline std::string convert_to_string( std::string const& obj)
 
 inline std::string convert_to_string( std::wstring const& s )
 {
- std::size_t len = wcslen( s.c_str() );
- char* c = reinterpret_cast<char*>( alloca( len ));
- wcstombs( c, s.c_str(), len );
+ std::mbstate_t state = std::mbstate_t();
+ const wchar_t* str = s.c_str();
+ const std::size_t len = std::wcsrtombs(nullptr, &str, 0, &state);
+ char* c = reinterpret_cast<char*>( alloca( len + 1));
+ wcstombs( c, s.c_str(), len + 1 );
 
  return std::string( c, c + len );
 }
@@ -80,7 +82,8 @@ inline char const* convert_to_native_string( const std::string& str )
 
 inline char const* convert_to_native_string( const wchar_t* str )
 {
- std::size_t len = wcslen( str ) + 1;
+ std::mbstate_t state = std::mbstate_t();
+ const std::size_t len = std::wcsrtombs(nullptr, &str, 0, &state) + 1;
  char* c = new char[len];
  wcstombs( c, str, len );
 
@@ -89,7 +92,9 @@ inline char const* convert_to_native_string( const wchar_t* str )
 
 inline char const* convert_to_native_string( std::wstring const& str )
 {
- std::size_t len = wcslen( str.c_str() ) + 1;
+ std::mbstate_t state = std::mbstate_t();
+ const wchar_t* wstr = str.c_str();
+ const std::size_t len = std::wcsrtombs(nullptr, &wstr, 0, &state) + 1;
  char* c = new char[len];
  wcstombs( c, str.c_str(), len );