Fix issue where iterating over an empty TSet or TOrderedSet results in a SIGSEVG

To reproduce bug:

```nimrod
import sets
var t: TSet[string]  # or TOrderedSet[string]
echo(t)
```

$ nimrod c -r b.nim 
config/nimrod.cfg(37, 2) Hint: added path: '/home/tjerk/.babel/pkgs/' [Path]
Hint: used config file '/home/tjerk/apps/Nimrod/config/nimrod.cfg' [Conf]
Hint: system [Processing]
Hint: b [Processing]
Hint: sets [Processing]
Hint: os [Processing]
Hint: strutils [Processing]
Hint: parseutils [Processing]
Hint: times [Processing]
Hint: posix [Processing]
Hint: hashes [Processing]
Hint: math [Processing]
gcc   -o /home/tjerk/dev/nimrod/bfair/b  /home/tjerk/dev/nimrod/bfair/nimcache/pure_math.o /home/tjerk/dev/nimrod/bfair/nimcache/pure_hashes.o /home/tjerk/dev/nimrod/bfair/nimcache/posix_posix.o /home/tjerk/dev/nimrod/bfair/nimcache/pure_times.o /home/tjerk/dev/nimrod/bfair/nimcache/pure_parseutils.o /home/tjerk/dev/nimrod/bfair/nimcache/pure_strutils.o /home/tjerk/dev/nimrod/bfair/nimcache/pure_os.o /home/tjerk/dev/nimrod/bfair/nimcache/collections_sets.o /home/tjerk/dev/nimrod/bfair/nimcache/Nimrod_system.o /home/tjerk/dev/nimrod/bfair/nimcache/bfair_b.o  -ldl -lm
Hint: operation successful (15733 lines compiled; 0.250 sec total; 22.225MB) [SuccessX]
/home/tjerk/dev/nimrod/bfair/b 
Traceback (most recent call last)
b.nim(5)                 b
sets.nim(40)             $
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Error: execution of an external program failed