ipv6: esp6: use BUG_ON instead of if condition followed by BUG

Use BUG_ON instead of if condition followed by BUG in esp_remove_trailer.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>