V4L/DVB (6305): V4L: videobuf-core.c avoid NULL dereferences in videobuf-core

The return value of videobuf_alloc() is unchecked but this function will
return NULL on an error.  Check for NULL and make videobuf_reqbufs()
return the number of successfully allocated buffers.

Also, fix saa7146_video.c and bttv-driver.c to use this returned
buffer count.

Tested against the vivi driver.  Not tested against saa7146 or bt8xx
devices.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
diff --git a/drivers/media/common/saa7146_video.c b/drivers/media/common/saa7146_video.c
index 29dbc60..f245a3b 100644
--- a/drivers/media/common/saa7146_video.c
+++ b/drivers/media/common/saa7146_video.c
@@ -1212,6 +1212,8 @@
 			mutex_unlock(&q->lock);
 			return err;
 		}
+
+		gbuffers = err;
 		memset(mbuf,0,sizeof(*mbuf));
 		mbuf->frames = gbuffers;
 		mbuf->size   = gbuffers * gbufsize;