readline: avoid memcpy() of overlapping regions

memcpy() for overlapping regions is undefined behavior; use memmove()
instead in readline_hist_add().

[Keep tab characters since surrounding code still uses them -- Stefan]

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

readline.c

@@ -248,8 +248,8 @@ static void readline_hist_add(ReadLineState *rs, const char *cmdline)
     if (idx == READLINE_MAX_CMDS) {
 	/* Need to get one free slot */
 	free(rs->history[0]);
-	memcpy(rs->history, &rs->history[1],
-	       (READLINE_MAX_CMDS - 1) * sizeof(char *));
+	memmove(rs->history, &rs->history[1],
+	        (READLINE_MAX_CMDS - 1) * sizeof(char *));
 	rs->history[READLINE_MAX_CMDS - 1] = NULL;
 	idx = READLINE_MAX_CMDS - 1;
     }