tcp: prevent div by zero in cc_htcp

Make sure the divident is at least one. While cwnd should
never be smaller than t_maxseg, this can happen during
Path MTU Discovery, or when TCP options are considered
in other parts of the stack.

PR:			276674
MFC after:		3 days
Reviewed By:		tuexen, #transport
Sponsored by:		NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D43797

(cherry picked from commit 38983d4)

sys/netinet/cc/cc_htcp.c

@@ -228,9 +228,9 @@ htcp_ack_received(struct cc_var *ccv, uint16_t type)
 				 * per RTT.
 				 */
 				CCV(ccv, snd_cwnd) += (((htcp_data->alpha <<
-				    HTCP_SHIFT) / (CCV(ccv, snd_cwnd) /
-				    CCV(ccv, t_maxseg))) * CCV(ccv, t_maxseg))
-				    >> HTCP_SHIFT;
+				    HTCP_SHIFT) / (max(1,
+				    CCV(ccv, snd_cwnd) / CCV(ccv, t_maxseg)))) *
+				    CCV(ccv, t_maxseg))  >> HTCP_SHIFT;
 		}
 	}
 }