udp: possible unaligned acces in gnrc_udp_hdr_build()

[daniel-k]

gnrc_pktsnip_t *gnrc_udp_hdr_build(gnrc_pktsnip_t *payload,
                                   uint8_t *src, size_t src_len,
                                   uint8_t *dst, size_t dst_len)

has uint8_t * arguments that will be dereferenced as uint16_t * which can lead to an unaligned access and the documentation doesn't warn the user.

[miri64]

I would prefer if the header build function would use the actual types BTW.

[miri64]

Fixed in #5118