Core: Use a shared_ptr for i18n categories.

This does not make them thread safe, but it reduces the chances of a crash
a bit (see #12594.)

Core/FileSystems/BlockDevices.cpp

@@ -64,7 +64,7 @@ u32 BlockDevice::CalculateCRC() {
 }
 
 void BlockDevice::NotifyReadError() {
-	I18NCategory *err = GetI18NCategory("Error");
+	auto err = GetI18NCategory("Error");
 	if (!reportedError_) {
 		host->NotifyUserMessage(err->T("Game disc read error - ISO corrupt"), 6.0f);
 		reportedError_ = true;