monitor.c: Fix infinite loop in monitor's auto-complete

The QEMU monitor enters an infinite loop when trying to auto-complete commands
that accept only optional parameters. The commands currently affected by this
issue are 'info registers' and 'info mtree'.
Reported-by: NDimitris Karagkasidis <t.pagef.lt@gmail.com>
Fixes: 48fe86f6Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180213125143.23488-1-dgilbert@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

monitor.c

@@ -3696,7 +3696,7 @@ static void monitor_find_completion_by_table(Monitor *mon,
 {
     const char *cmdname;
     int i;
-    const char *ptype, *str, *name;
+    const char *ptype, *old_ptype, *str, *name;
     const mon_cmd_t *cmd;
     BlockBackend *blk = NULL;
 
@@ -3741,7 +3741,9 @@ static void monitor_find_completion_by_table(Monitor *mon,
             }
         }
         str = args[nb_args - 1];
-        while (*ptype == '-' && ptype[1] != '\0') {
+        old_ptype = NULL;
+        while (*ptype == '-' && old_ptype != ptype) {
+            old_ptype = ptype;
             ptype = next_arg_type(ptype);
         }
         switch(*ptype) {