net: lib: dns: Check return value of net_context_recv()

Given that K_NO_WAIT is passed as a timeout to net_context_recv(), it's
unlikely this function will return any error value.  It's cheap to
check, though, so do it.

Coverity-CID: 170580
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>