Fix errata output error caused by race condition

`this.platforms` is a computed value and may be empty when
`platformName()` is executed, which can result in accessing a
non-existent `label` property.

This fix unsure that matched platform exists before accessing it,
preventing UI breakage.

Fixes: AlmaLinux/build-system#430

src/components/ErrataInfo.vue

@@ -815,7 +815,8 @@
         return description && description !== this.advisory.original_description
       },
       platformName(id) {
-        return this.platforms.find((platform) => platform.value == id).label
+        const platform = this.platforms.find((platform) => platform.value == id)
+        return platform ? platform.label : ''
       },
       cveRows(refs) {
         return refs.filter((ref) => ref.cve)

src/pages/ErrataFeed.vue

@@ -532,7 +532,8 @@
         return advisory.title ? advisory.title : advisory.original_title
       },
       platformName(id) {
-        return this.platforms.find((platform) => platform.value == id).label
+        const platform = this.platforms.find((platform) => platform.value == id)
+        return platform ? platform.label : ''
       },
     },
     components: {