net: lwm2m: Fix PULL FW update in case of URI parse errors

The memset on firmware_ctx during PULL FW update initialization will
set the socket descriptor to a valid value of 0. This leads to an error
if parsing of the URI provided by the server fails, and the firware_ctx
is closed - the socket with a descriptor 0 will be accidently closed.
Fix this by invalidating the socket FD after the memset on
initialization.

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