Fix inaccurate dragon fire damage with shield equipped (#1234)

What have you done in this MR?

• Fixed the incorrect damage reduction for dragon fire when wearing a dragonfire-reduction shield.
• This is just a quick fix for (closes #1234 (closed)). Due to unclear and sometimes conflicting historical sources, it's likely there's more inaccuracies in terms of dragon fire damage and the interactions with dragon fire damage reduction.

Are there any tricky things testers should keep an eye out for?

• No
• I have tested these changes thoroughly.
• I acknowledge that this contribution will be released under the AGPL license.