net: lwm2m: Fix string truncation on some content formatters

LwM2M engine does not allow strings to be truncated as it would
lead to invalind UTF-8 strings.

However, some content formatters were truncating, so I fixed at least
these
* plain text
* TLV
* SenML CBOR
* SenML JSON
* JSON
* CBOR

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>