[assimp] Make sure ctype calls use unsigned char.

Cast to unsigned char as required by C++ (see C++ **[cctype.cyn]** -> ISO C99 section 7.4, [see also](https://en.cppreference.com/w/cpp/string/byte/isspace)).

Addresses https://github.com/assimp/assimp/issues/3867 and then some.

code/AssetLib/X/XFileImporter.cpp

@@ -667,8 +667,8 @@ void XFileImporter::ConvertMaterials( aiScene* pScene, std::vector<XFile::Materi
 
                 // convert to lower case for easier comparison
                 for ( unsigned int c = 0; c < sz.length(); ++c ) {
-                    if ( isalpha( sz[ c ] ) ) {
-                        sz[ c ] = (char) tolower( sz[ c ] );
+                    if ( isalpha( (unsigned char) sz[ c ] ) ) {
+                        sz[ c ] = (char) tolower( (unsigned char) sz[ c ] );
                     }
                 }