putgrent: Stop writing output on first failure

This way, if an fprintf fails, we get an incomplete group entry rather
than a corrupted one.

src/passwd/putgrent.c

@@ -6,9 +6,10 @@ int putgrent(const struct group *gr, FILE *f)
 	int r;
 	size_t i;
 	flockfile(f);
-	r = fprintf(f, "%s:%s:%d:", gr->gr_name, gr->gr_passwd, gr->gr_gid);
+	if ((r = fprintf(f, "%s:%s:%d:", gr->gr_name, gr->gr_passwd, gr->gr_gid))<0) goto done;
 	if (gr->gr_mem) for (i=0; gr->gr_mem[i]; i++)
-		if (fprintf(f, "%s%s", i?",":"", gr->gr_mem[i])<0) r = -1;
+		if ((r = fprintf(f, "%s%s", i?",":"", gr->gr_mem[i]))<0) goto done;
+done:
 	funlockfile(f);
 	return r<0 ? -1 : 0;
 }