Explicitly initialize the memory buffer to store O_ICMP6TYPE opcode.

By default next_cmd() initializes only first u32 of opcode. O_ICMP6TYPE
opcode has array of bit masks to store corresponding ICMPv6 types.
An opcode that precedes O_ICMP6TYPE, e.g. O_IP6_DST, can have variable
length and during opcode filling it can modify memory that will be used
by O_ICMP6TYPE opcode. Without explicit initialization this leads to
creation of wrong opcode.

Reported by:	Boris N. Lytochkin
Obtained from:	Yandex LLC
MFC after:	3 days

sbin/ipfw/ipv6.c

@@ -143,6 +143,7 @@ fill_icmp6types(ipfw_insn_icmp6 *cmd, char *av, int cblen)
        uint8_t type;
 
        CHECK_LENGTH(cblen, F_INSN_SIZE(ipfw_insn_icmp6));
+       memset(cmd, 0, sizeof(*cmd));
        while (*av) {
 	       if (*av == ',')
 		       av++;