V4L/DVB (6581): Fix: avoids negative vma usage count

Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/em28xx/em28xx-video.c

@@ -571,6 +571,8 @@ static void em28xx_vm_close(struct vm_area_struct *vma)
 {
 	/* NOTE: buffers are not freed here */
 	struct em28xx_frame_t *f = vma->vm_private_data;
+
+	if (f->vma_use_count)
 		f->vma_use_count--;
 }