[css-flexbox-2] Catch the error case where a single overflowing item is followed by a zero-size item.

Author: tabatkins

css-flexbox-2/Overview.bs

@@ -4182,13 +4182,15 @@ Balancing Flex Items</h3>
 		into exactly |line count| numbers of contiguous sequences,
 		under the following conditions:
 
+		* At least one [=flex item=] is assigned to each sequence.
 		* The sum of the [=flex item=] sizes
 			of all the [=flex items=] in the sequence
 			(the |line size|)
-			do not exceed the [=inner size|inner=] [=main size=] of the [=flex container=].
-		* At least one [=flex item=] is assigned to each sequence.
+			do not exceed the [=inner size|inner=] [=main size=] of the [=flex container=]
+			(unless it's a single overflowing item).
 		* If a [=flex item's=] size is zero,
-			and it could be assigned either to the end of a preceding sequence
+			the preceding sequence was not overflowing,
+			and it could be assigned either to the end of the preceding sequence
 			or the beginning of the next sequence,
 			it is assigned to the end of the preceding sequence.
 		* Calling the difference between the |line size|