Collada exporter: Fix isalnum(negative number) assert.

isalnum(negative number) asserts in MSVC.
It was first visible problem when exporting unicode filenames.

code/ColladaExporter.cpp

@@ -540,7 +540,7 @@ void ColladaExporter::WriteImageEntry( const Surface& pSurface, const std::strin
     std::stringstream imageUrlEncoded;
     for( std::string::const_iterator it = pSurface.texture.begin(); it != pSurface.texture.end(); ++it )
     {
-      if( isalnum( *it) || *it == '_' || *it == '.' || *it == '/' || *it == '\\' )
+      if( isalnum( (unsigned char) *it) || *it == '_' || *it == '.' || *it == '/' || *it == '\\' )
         imageUrlEncoded << *it;
       else
         imageUrlEncoded << '%' << std::hex << size_t( (unsigned char) *it) << std::dec;