Cleanup: IDTypeInfo new asset_type_info member.

Two issues addressed here:

I) `asset_type_info` is sub-data, not a callback. Therefore, move it
before the callbacks in the `IDTypeInfo` struct.

II) More important, initialize this new attribute in *ALL* `IDTypeInfo`
instances. No member of this struct should ever be left implicitely
uninitilazed, ever.

Aftermath of rBa84f1c02d251.

source/blender/blenkernel/intern/speaker.c

@@ -99,6 +99,7 @@ IDTypeInfo IDType_ID_SPK = {
     .name_plural = "speakers",
     .translation_context = BLT_I18NCONTEXT_ID_SPEAKER,
     .flags = IDTYPE_FLAGS_APPEND_IS_REUSABLE,
+    .asset_type_info = NULL,
 
     .init_data = speaker_init_data,
     .copy_data = NULL,