[css-flexbox-1] Fix divide-by-zero.

Author: tabatkins

css-flexbox-1/Overview.bs

@@ -2908,9 +2908,11 @@ Flex Container Intrinsic Main Sizes</h4>
 			For each <a>flex item</a>,
 			subtract its outer <a>flex base size</a> from its [[#intrinsic-item-contributions|max-content contribution]] size.
 			If that result is positive,
-			divide by its <a>flex grow factor</a>;
+			divide by its <a>flex grow factor</a>
+			(if dividing by zero, treat the result as infinity);
 			if negative,
-			divide by its <a>scaled flex shrink factor</a>.
+			divide by its <a>scaled flex shrink factor</a>
+			(if dividing by zero, treat the result as negative infinity).
 			This is the item's <var>desired flex fraction</var>.
 
 		<li>