Base SIZE_T_CEILING on SSIZE_T_MAX.

src/common/torint.h

@@ -331,7 +331,7 @@ typedef uint32_t uintptr_t;
 #endif
 
 /* Any size_t larger than this amount is likely to be an underflow. */
-#define SIZE_T_CEILING (sizeof(char)<<(sizeof(size_t)*8 - 1))
+#define SIZE_T_CEILING (SSIZE_T_MAX-16)
 
 #endif /* __TORINT_H */