GUACAMOLE-2259: Set initial height/width on last_frame to 0

[sschiffli]

This ensures the check for layer size changes will send the proper size instruction even if the layer happens to be GUAC_DISPLAY_RESIZE_FACTOR x GUAC_DISPLAY_RESIZE_FACTOR (currently 64x64)

/* Commit any change in layer size */
if (current->pending_frame.width != current->last_frame.width
        || current->pending_frame.height != current->last_frame.height) {

Having the last_frame buffer initially NULL also more clearly conveys that no frame has been sent yet, and once a frame has been sent, the buffer reallocates on sizes changes and will function normally.