page-xfer: handle a case when splice returns zero

A return value of 0 means end of input, so we need to
stop reading from this descriptor.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/page-xfer.c

@@ -222,6 +222,10 @@ static int write_pages_loc(struct page_xfer *xfer,
 			pr_perror("Unable to spice data");
 			return -1;
 		}
+		if (ret == 0) {
+			pr_err("A pipe was closed unexpectedly");
+			return -1;
+		}
 		curr += ret;
 		if (curr == len)
 			break;
@@ -626,6 +630,10 @@ static int page_server_add(int sk, struct page_server_iov *pi)
 			pr_perror("Can't read from socket");
 			return -1;
 		}
+		if (chunk == 0) {
+			pr_err("A socket was closed unexpectedly");
+			return -1;
+		}
 
 		if (lxfer->write_pages(lxfer, cxfer.p[0], chunk))
 			return -1;