try wrapping your next button with an if, untested but youll get the idea

something like

{if $item->id+1}

<input style='float:right' type="button" class="next-btn" name="b2" value="next" onclick="location.href='/item/view/{$item->id+1}'">

{/if}

you may have to qualify it but not sure something like

{if $item->id+1 eq whatever}  whatever that value might be, null 0 im not sure what it would be in your case.

in smarty you can use  eq for =   ne for !=   gt  for >   amd   lt for < 

equals notequals greaterthan lessthan