readline: Fix buffer overrun on re-add to history

readline_hist_add() moves the history entry to the end of history.  It
uses memmove() to move rs->history[idx + 1..] to rs->history[idx..].
However, its size argument is off by two array elements, so it writes
one element beyond rs->history[], and reads two.

On my system, this clobbers rs->hist_entry and the hole right after
it.  Since the function assigns to rs->hist_entry in time, the bug has
no ill effects for me.

Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

readline.c

@@ -236,7 +236,7 @@ static void readline_hist_add(ReadLineState *rs, const char *cmdline)
 	    new_entry = hist_entry;
 	    /* Put this entry at the end of history */
 	    memmove(&rs->history[idx], &rs->history[idx + 1],
-		    (READLINE_MAX_CMDS - idx + 1) * sizeof(char *));
+		    (READLINE_MAX_CMDS - (idx + 1)) * sizeof(char *));
 	    rs->history[READLINE_MAX_CMDS - 1] = NULL;
 	    for (; idx < READLINE_MAX_CMDS; idx++) {
 		if (rs->history[idx] == NULL)