Fix type of iterator variable in SH_START_ITERATE

Also add comment to make the reasoning behind the Assert() more explicit (per
Tom).

Reported-by: Ranier Vilela
Discussion: https://postgr.es/m/CAEudQAocXNJ6s1VLz+hMamLAQAiewRoW17OJ6-+9GACKfj6iPQ@mail.gmail.com
Backpatch: 11-

src/include/lib/simplehash.h

@@ -965,7 +965,6 @@ SH_DELETE_ITEM(SH_TYPE * tb, SH_ELEMENT_TYPE * entry)
 SH_SCOPE void
 SH_START_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter)
 {
-	int			i;
 	uint64		startelem = PG_UINT64_MAX;
 
 	/*
@@ -973,7 +972,7 @@ SH_START_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter)
 	 * supported, we want to start/end at an element that cannot be affected
 	 * by elements being shifted.
 	 */
-	for (i = 0; i < tb->size; i++)
+	for (uint32 i = 0; i < tb->size; i++)
 	{
 		SH_ELEMENT_TYPE *entry = &tb->data[i];
 
@@ -984,6 +983,7 @@ SH_START_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter)
 		}
 	}
 
+	/* we should have found an empty element */
 	Assert(startelem < SH_MAX_SIZE);
 
 	/*