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: NBrandon Philips <bphilips@suse.de>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>