media: dmxdev: change the check for problems allocing secfeed

While the logic there is right, it tricks static check analyzers,
like smatch:

	drivers/media/dvb-core/dmxdev.c:729 dvb_dmxdev_filter_start() error: we previously assumed '*secfeed' could be null (see line 719)

Because the implementation of the filter itself is made via
a callback, with its real implementation at the
dvbdmx_allocate_section_feed() inside dvb_demux.c.

So, change the check logic to make it clear that the function
will not try to use *secfeed == NULL.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>