Fix handling of negative flex gap (#1405)

Summary:
X-link: https://github.com/facebook/react-native/pull/39596

Pull Request resolved: https://github.com/facebook/yoga/pull/1405

I noticed that we weren't clamping negative flex gap values to zero. This fixes that bug.

Reviewed By: rshest

Differential Revision: D49530494

fbshipit-source-id: 069db7312f72a085c5c4b01ead7bc66a353a07e5

gentest/gentest.js

@@ -701,7 +701,7 @@ function calculateTree(root, roundToPixelGrid) {
 
 function getYogaStyle(node) {
   // TODO: Relying on computed style means we cannot test shorthand props like
-  // "padding", "margin", "gap".
+  // "padding", "margin", "gap", or negative values.
   return [
     'direction',
     'flex-direction',