net: coap: Fix error handling for coap option length

Currently, we check the length of an option value in the
coap_packet_append_option() function.  This isn't required as
we're appending to a net_pkt and not using struct coap_option
where the limitation is imposed.

Instead, we should check the option value length in
parse_option() where we assign the value to a struct
coap_option.

Signed-off-by: Michael Scott <michael.scott@linaro.org>