ed: Reset modflag in clearbuf()

For the same reason we don't care about the state of
the buffer in clearundo() we can reset modflag in
clearbuf(), because after calling it we don't want
to know anything about the previous state.

ed.c

@@ -322,7 +322,7 @@ clearbuf()
 	free(zero);
 	zero = NULL;
 	scratch = csize = idxsize = lastidx = curln = lastln = 0;
-	lastln = curln = 0;
+	modflag = lastln = curln = 0;
 }
 
 static void