UI: Fix path calculation for disk space check

When using custom FFmpeg output mode, the check would instead use the
standard recording path which is no longer visible in the settings. This
commit also simplifies the checks by moving the duplicated code to a new
function.

UI/window-basic-stats.cpp

@@ -297,13 +297,7 @@ void OBSBasicStats::Update()
 
 	/* ------------------ */
 
-	const char *mode = config_get_string(main->Config(), "Output", "Mode");
-	const char *path = strcmp(mode, "Advanced")
-				   ? config_get_string(main->Config(),
-						       "SimpleOutput",
-						       "FilePath")
-				   : config_get_string(main->Config(), "AdvOut",
-						       "RecFilePath");
+	const char *path = main->GetCurrentOutputPath();
 
 #define MBYTE (1024ULL * 1024ULL)
 #define GBYTE (1024ULL * 1024ULL * 1024ULL)