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