fix for allocations over 2gig crashing blender (even on 64 bit systems)

the memheader len would wrap to a negative number and when freeing it would write into the memheader.

intern/guardedalloc/intern/mallocn.c

@@ -82,7 +82,7 @@ typedef struct localListBase
 	/* note: keep this struct aligned (e.g., irix/gcc) - Hos */
 typedef struct MemHead {
 	int tag1;
-	int len;
+	unsigned int len;
 	struct MemHead *next,*prev;
 	const char * name;
 	const char * nextname;