ethumbd - fix possible out of bounds access to work table

this should fix CID 1039314 in case an unsused table slot cannot be
found. this should actualyl never happen because table expands/allocs
zero out new entires so they are "unused".

src/bin/ethumb_client/ethumbd.c

@@ -811,6 +811,12 @@ _ethumb_table_append(Ethumbd *ed)
 	  break;
      }
 
+   if (i >= q->max_count)
+     {
+        CRIT("cannot find free table slot in table of %i", q->max_count);
+        return -1;
+     }
+
    snprintf(buf, sizeof(buf), "%s/%d", _ethumb_dbus_path, i);
    q->table[i].used = 1;
    q->table[i].path = eina_stringshare_add(buf);