Use correct way to check if we logged in yet or not.

Without this fix, we trigger an assert, in debug mode, that was added
to Singularity exactly to find out if we called functions like
getExpandedFilename(LL_PATH_PER_SL_ACCOUNT ...) before logging in.
Checking if THAT function returns empty() is clearly not safe, but
very error prone.

indra/newview/llviewertexturelist.cpp

@@ -277,7 +277,7 @@ void LLViewerTextureList::shutdown()
 			break;
 	}
 	
-	if (count > 0 && !gDirUtilp->getExpandedFilename(LL_PATH_PER_SL_ACCOUNT, "").empty())
+	if (count > 0 && !gDirUtilp->getLindenUserDir(true).empty())
 	{
 		std::string filename = gDirUtilp->getExpandedFilename(LL_PATH_PER_SL_ACCOUNT, get_texture_list_name());
 		llofstream file;