bpv6: Fix possible infinite loop

display_extension_block is supposed to return the current offset,
not the number of bytes remaining. The number of bytes remaining
can be less than the current offset and cause an infinite loop.

In the case of an error, set lastheader and return the current
offset in order to break out of the main processing loop.

Fix #18711.

epan/dissectors/packet-bpv6.c

@@ -1704,8 +1704,8 @@ display_extension_block(proto_tree *tree, tvbuff_t *tvb, packet_info *pinfo, int
             proto_tree_add_int(result_tree, hf_block_ciphersuite_result_item_length, tvb, offset, sdnv_length, result_item_length);
             if (ei) {
                 proto_tree_add_expert(result_tree, pinfo, ei, tvb, offset, -1);
-                offset = tvb_reported_length_remaining(tvb, offset);
-                break;
+                *lastheader = TRUE;
+                return offset;
             }
             offset += sdnv_length;