fix: Wrong buffer size in path string conversion functions (#746)

Fixes #733

include/boost/gil/io/path_spec.hpp

@@ -1,5 +1,6 @@
 //
 // Copyright 2007-2008 Andreas Pokorny, Christian Henning
+// Copyright 2024 Dirk Stolle
 //
 // Distributed under the Boost Software License, Version 1.0
 // See accompanying file LICENSE_1_0.txt or copy at
@@ -11,6 +12,7 @@
 #include <boost/gil/io/detail/filesystem.hpp>
 
 #include <cstdlib>
+#include <cwchar>
 #include <string>
 #include <type_traits>
 
@@ -41,11 +43,13 @@ 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);
+    std::string result(len, '\0');
+    std::wcstombs( &result[0], s.c_str(), len );
 
-    return std::string( c, c + len );
+    return result;
 }
 
 inline std::string convert_to_string( char const* str )
@@ -80,18 +84,21 @@ 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 );
+    std::wcstombs( c, str, len );
 
     return c;
 }
 
 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 );
+    std::wcstombs( c, str.c_str(), len );
 
     return c;
 }