drizzle_state_field_read() can read past packet boundary

drizzle_state_field_read() has logic like this in a couple of places:

 [ http://bazaar.launchpad.net/~drizzle-trunk/libdrizzle/libdrizzle-redux/view/99.1.7/libdrizzle/field.cc#L234 ]

    if ((size_t)(con->buffer_size) >= con->result->field_total)
    {
      con->result->field_size= con->result->field_total;
    }
    else
    {
      con->result->field_size= con->buffer_size;
    }

However, if the field is broken across multiple packets, and the buffer contains the rest of this packet and the beginning of the next, this can read too much data into the field (including the packet header of the next packet). The field_size should be set to the smallest size among buffer_size, packet_size, and field_total.