decode/tcp: allow 4 byte TFO with 2 byte cookie

(cherry picked from commit f5bd55dac8)

src/decode-tcp.h

@@ -63,7 +63,7 @@
 #define TCP_OPT_MSS_LEN                      4
 #define TCP_OPT_SACK_MIN_LEN                 10 /* hdr 2, 1 pair 8 = 10 */
 #define TCP_OPT_SACK_MAX_LEN                 34 /* hdr 2, 4 pair 32= 34 */
-#define TCP_OPT_TFO_MIN_LEN                  6  /* kind, len, 6 */
+#define TCP_OPT_TFO_MIN_LEN                  4  /* kind, len, 2 bytes cookie: 4 */
 #define TCP_OPT_TFO_MAX_LEN                  18 /* kind, len, 18 */
 
 /** Max valid wscale value. */