fix(Xapi#_installPoolPatchAndRequirements): ignore required patch if already installed (#564)

src/xapi/mixins/patching.js

@@ -380,10 +380,14 @@ export default {
 
     if (requirements.length) {
       for (const requirementUuid of requirements) {
-        await this._installPoolPatchAndRequirements(patchesByUuid[requirementUuid], patchesByUuid, host)
+        const requirement = patchesByUuid[requirementUuid]
+
+        if (requirement != null) {
+          await this._installPoolPatchAndRequirements(requirement, patchesByUuid, host)
           host = host && this.getObject(host.$id)
         }
       }
+    }
 
     return host == null
       ? this.installPoolPatchOnAllHosts(patch.uuid)