Fix possible crash in file download after error.

Regression was introduced in 2fa2fa41c5.

In file download failed handler we suggest to try to load the file
once again to the same location. After some changes we started to
forget filename before failed handler. That resulted in large files
loading to memory instead of hard drive.

Add a precondition in FileLoader to prevent such bugs in the future.

Telegram/SourceFiles/config.h

@@ -93,13 +93,9 @@ enum {
 	AudioVoiceMsgUpdateView = 100, // 100ms
 	AudioVoiceMsgChannels = 2, // stereo
 	AudioVoiceMsgBufferSize = 256 * 1024, // 256 Kb buffers (1.3 - 3.0 secs)
-	AudioVoiceMsgInMemory = 2 * 1024 * 1024, // 2 Mb audio is hold in memory and auto loaded
 
-	StickerInMemory = 2 * 1024 * 1024, // 2 Mb stickers hold in memory, auto loaded and displayed inline
 	StickerMaxSize = 2048, // 2048x2048 is a max image size for sticker
 
-	AnimationInMemory = 10 * 1024 * 1024, // 10 Mb gif and mp4 animations held in memory while playing
-
 	MaxZoomLevel = 7, // x8
 	ZoomToScreenLevel = 1024, // just constant