net: lwm2m: Fix JSON write handling

There were several issues preventing JSON format writes to work
correctly:

1. The formatter wrongly assumed that Base Name and Relative Name values
   read from the message are NULL terminated, which in result could give
   invalid results when combining them.
2. The formatter wrongly assumed that Relative Name is always present,
   which is not always the case. In result, it failed to parse messages,
   which contained full path in their Base Name Field
3. There were no boundaries check when reading JSON variable name/value,
   which could lead to buffer overflow in case malformed data was
   received.

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