Fix for memory leak caused by re-making mipmaps

Problem was caused by different limits for "last" mipmap: it was 1x1
for mkaing mipmaps and 2x2 for re-making.

2x2 should be enough for mipmap.

source/blender/imbuf/intern/filter.c

@@ -437,7 +437,7 @@ void IMB_makemipmap(ImBuf *ibuf, int use_filter)
 		hbuf= ibuf->mipmap[curmap];
 		hbuf->miplevel= curmap+1;
 
-		if(!hbuf || (hbuf->x == 1 && hbuf->y == 1))
+		if(!hbuf || (hbuf->x <= 2 && hbuf->y <= 2))
 			break;
 
 		curmap++;