[Patch] Fix bounds check in trim_array()

Hi,

while working on something else i encountered a bug in the trim_array() 
function. The bounds check fails for empty arrays without any 
dimensions. It reads the size of the non existing first dimension to 
determine the arrays length.

   select trim_array('{}'::int[], 10);
   ------------
    {}

   select trim_array('{}'::int[], 100);
   ERROR:  number of elements to trim must be between 0 and 64

The attached patch fixes that check.

Martin