Windows: Don't remove file associations if they aren't our own.

src/preferences/preferences.h

@@ -1248,7 +1248,7 @@ public:
       settings.setValue("qBittorrent/shell/open/command/Default", command_str);
       settings.setValue("qBittorrent/Content Type/Default", "application/x-bittorrent");
       settings.setValue("qBittorrent/DefaultIcon/Default", icon_str);
-    } else {
+    } else if (isTorrentFileAssocSet()) {
       settings.remove(".torrent/Default");
       settings.remove(".torrent/Content Type");
       settings.remove("qBittorrent/shell/Default");
@@ -1272,7 +1272,7 @@ public:
       settings.setValue("Magnet/DefaultIcon/Default", icon_str);
       settings.setValue("Magnet/shell/Default", "open");
       settings.setValue("Magnet/shell/open/command/Default", command_str);
-    } else {
+    } else if (isMagnetLinkAssocSet()) {
       settings.remove("Magnet/Default");
       settings.remove("Magnet/Content Type");
       settings.remove("Magnet/URL Protocol");