[media] dvb: avoid warning in dvb_net

With gcc-5 or higher on x86, we can get a bogus warning in the
dvb-net code:

drivers/media/dvb-core/dvb_net.c: In function 'dvb_net_ule':
arch/x86/include/asm/string_32.h:78:22: error: '*((void *)&dest_addr+4)' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The problem here is that gcc doesn't track all of the conditions
to prove it can't end up copying uninitialized data.
This changes the logic around so we zero out the destination
address earlier when we determine that it is not set here.
This allows the compiler to figure it out.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>