cmd/ld: skip 0-length write in cwrite

The 0-length part is fine, but some callers that write 0 bytes
also pass nil as the data pointer, and the Plan 9 kernel kills the
process with 'invalid address in sys call' in that case.

R=ken2
CC=golang-dev
https://golang.org/cl/6862051

src/cmd/ld/lib.c

@@ -1493,6 +1493,8 @@ void
 cwrite(void *buf, int n)
 {
 	cflush();
+	if(n <= 0)
+		return;
 	if(write(cout, buf, n) != n) {
 		diag("write error: %r");
 		errorexit();