Fix C++ slint_register_font_from_* functions regarding to the error…

… string

The C++ code already instentiate a `SharedString` on the stack, we are
not supposed to ptr::write into it.
Lickyly this didn't have any effect because the default constructed
string can be leaked without problem, but better be correct.

Also aboud temporary String allocation

api/cpp/lib.rs

@@ -104,34 +104,30 @@ pub unsafe extern "C" fn slint_quit_event_loop() {
 pub unsafe extern "C" fn slint_register_font_from_path(
     win: *const WindowAdapterRcOpaque,
     path: &SharedString,
-    error_str: *mut SharedString,
+    error_str: &mut SharedString,
 ) {
     let window_adapter = &*(win as *const Rc<dyn WindowAdapter>);
-    core::ptr::write(
-        error_str,
-        match window_adapter.renderer().register_font_from_path(std::path::Path::new(path.as_str()))
-        {
-            Ok(()) => Default::default(),
-            Err(err) => err.to_string().into(),
-        },
-    )
+    *error_str = match window_adapter
+        .renderer()
+        .register_font_from_path(std::path::Path::new(path.as_str()))
+    {
+        Ok(()) => Default::default(),
+        Err(err) => i_slint_core::string::ToSharedString::to_shared_string(&err),
+    };
 }
 
 #[cfg(feature = "std")]
 #[no_mangle]
 pub unsafe extern "C" fn slint_register_font_from_data(
     win: *const WindowAdapterRcOpaque,
     data: i_slint_core::slice::Slice<'static, u8>,
-    error_str: *mut SharedString,
+    error_str: &mut SharedString,
 ) {
     let window_adapter = &*(win as *const Rc<dyn WindowAdapter>);
-    core::ptr::write(
-        error_str,
-        match window_adapter.renderer().register_font_from_memory(data.as_slice()) {
-            Ok(()) => Default::default(),
-            Err(err) => err.to_string().into(),
-        },
-    )
+    *error_str = match window_adapter.renderer().register_font_from_memory(data.as_slice()) {
+        Ok(()) => Default::default(),
+        Err(err) => i_slint_core::string::ToSharedString::to_shared_string(&err),
+    };
 }
 
 #[no_mangle]