warn if written size is different to original one

This patch also eliminates compiler warning:
 yavta.c: In function ‘video_save_image’:
 yavta.c:974:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Print different messages depending on whether the return value is
negative or positive]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
diff --git a/yavta.c b/yavta.c
index b5b4487..2636577 100644
--- a/yavta.c
+++ b/yavta.c
@@ -1006,6 +1006,12 @@
 
 	ret = write(fd, dev->buffers[buf->index].mem, buf->bytesused);
 	close(fd);
+
+	if (ret < 0)
+		printf("write error: %s (%d)\n", strerror(errno), errno);
+	else if (ret != (int)buf->bytesused)
+		printf("write error: only %d bytes written instead of %u\n",
+		       ret, buf->bytesused);
 }
 
 static int video_do_capture(struct device *dev, unsigned int nframes,