pidl: Don't generate variables declarations for pointless array counters.

pidl: Don't generate variables declarations for pointless array counters.

Code isn't generated to iterate over arrays of length 0, but the
variable declarations still are.  The result is 'unused variable'
warnings. This only seems to be happening in one place right now, so I
targeted the fix to this case, but refactoring could be done to make
all variable declarations do this zero length check.  Making it the
default would be a much more invasive fix.

Jelmer, please check!