net: lwm2m: Make sure that endpoint string is NULL terminated

In case the endpoint string provided by the application is longer or
equal to CLIENT_EP_LEN - 1, the strncpy() function will not add the NULL
terminator. As the endpoint buffer is treated as a C-string in other
places in the code, make sure it's NULL terminated by adding NULL
explicitly at the end of the buffer.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>