Avoid undefined behaviour in PACKET_buf_init

Change the sanity check in PACKET_buf_init to check for excessive length
buffers, which should catch the interesting cases where len has been cast
from a negative value whilst avoiding any undefined behaviour.

RT#4094
Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/packet_locl.h

@@ -111,7 +111,7 @@ __owur static inline int PACKET_buf_init(PACKET *pkt, unsigned char *buf,
                                          size_t len)
 {
     /* Sanity check for negative values. */
-    if (buf + len < buf)
+    if (len > (size_t)(SIZE_MAX / 2))
         return 0;
 
     pkt->curr = buf;