decode-ipv6: Set IPv6 proto incase of ext header parsing error

Set the IPv6 packet proto before parsing the ext headers, similar to
decode-ipv4, incase of an ext header parsing error. Otherwise
rule decode-events are not triggered for packets encapsulated in IPv6.

Bug: #6086.

src/decode-ipv6.c

@@ -574,6 +574,7 @@ int DecodeIPV6(ThreadVars *tv, DecodeThreadVars *dtv, Packet *p, const uint8_t *
         CLEAR_IPV6_PACKET(p);
         return TM_ECODE_FAILED;
     }
+    p->proto = IPV6_GET_NH(p);
 
 #ifdef DEBUG
     if (SCLogDebugEnabled()) { /* only convert the addresses if debug is really enabled */