fix: prevent TypeErrors in color-contrast checks (#1320)

This patch adds a safeguard to `color.filteredRectStack` which prevents a `TypeError: Cannot read property '0' of undefined` from being thrown.

This is not a perfect solution, but a stop-gap instead. It's still unclear why we're getting `undefined` here. More investigation is needed, but this can happen at a later date.

Closes #1306 and #1259.

lib/commons/color/get-background-color.js

@@ -284,6 +284,14 @@ color.filteredRectStack = function(elm) {
 			let rectA = rectStack[index - 1],
 				rectB = rectStack[index];
 
+			// If either rect are undefined, we cannot complete the test. We exit
+			// early to avoid unhelpful errors.
+			// See https://github.com/dequelabs/axe-core/issues/1306.
+			if (rectA === undefined || rectB === undefined) {
+				isSame = false;
+				return;
+			}
+
 			// if elements in clientRects are the same
 			// or the boundingClientRect contains the differing element, pass it
 			isSame =