tool_urlglob.c: added some comments to clarify for loop conditions

I was tempted to change those to >= 0 until I saw that this is
actually a for loop that terminates once i underflows.

src/tool_urlglob.c

@@ -465,6 +465,7 @@ void glob_cleanup(URLGlob* glob)
   size_t i;
   int elem;
 
+  /* the < condition is required since i underflows! */
   for(i = glob->size - 1; i < glob->size; --i) {
     if((glob->pattern[i].type == UPTSet) &&
        (glob->pattern[i].content.Set.elements)) {
@@ -498,6 +499,7 @@ int glob_next_url(char **globbed, URLGlob *glob)
 
     /* implement a counter over the index ranges of all patterns,
        starting with the rightmost pattern */
+    /* the < condition is required since i underflows! */
     for(i = glob->size - 1; carry && (i < glob->size); --i) {
       carry = FALSE;
       pat = &glob->pattern[i];