Update smooth_1.php

Hi,

I've just been doing some debugging and came across this; hope it helps. It worked for me :)

If default css is not used, 'undefined index' errors can occur because the array values for $smooth_slider_css are defined in a conditional. If the $default condition is not met, multiple errors will occur for each item in the loop because the referenced values have not been defined

This should be a simple fix:
Setting the values to empty when the array is first assigned, outside of the conditional, prevents the errors and shouldn't affect any existing functionality.(Line 626)