(ef_ensure_space): New.

(edit): Use it to to fix copying to virgin unit slot.  Closes #896475.
From Ron Koenderink.

src/lib/common/file.c

@@ -474,3 +474,13 @@ ef_check(int type)
     }
     return 0;
 }
+
+int
+ef_ensure_space(int type, int id, int count)
+{
+    while (id >= empfile[type].fids) {
+	if (!ef_extend(type, count))
+	    return 0;
+    }
+    return 1;
+}