Fix crash bug if V3 config had > 1 destinations

config/config_test.go

@@ -102,6 +102,66 @@ profiles:
 	os.Remove(cs.ConfigPath)
 }
 
+func TestMigrateV3toV4CrashBug(t *testing.T) {
+	v3Config := `config_version: 3
+server:
+  port: 6123
+  address: https://superaddress.here.com
+  download_path: /home/path/gropple
+  maximum_active_downloads_per_domain: 2
+ui:
+  popup_width: 500
+  popup_height: 500
+destinations:
+- name: somegifs
+  path: /home/path/somegifs
+- name: otherstuff
+  path: /home/path/otherstuff
+profiles:
+- name: standard video
+  command: yt-dlp
+  args:
+  - --newline
+  - --write-info-json
+  - -f
+  - bestvideo[ext=mp4]+bestaudio[ext=m4a]/best[ext=mp4]/best
+  - --verbose
+  - --embed-metadata
+  - --embed-subs
+  - --embed-thumbnail
+- name: standard mp3
+  command: yt-dlp
+  args:
+  - --extract-audio
+  - --audio-format
+  - mp3
+  - --prefer-ffmpeg
+`
+	cs := configServiceFromString(v3Config)
+	err := cs.LoadConfig()
+	if err != nil {
+		t.Errorf("got error when loading config: %s", err)
+	}
+	if cs.Config.ConfigVersion != 4 {
+		t.Errorf("did not migrate version (it is '%d')", cs.Config.ConfigVersion)
+	}
+	if cs.Config.Server.MaximumActiveDownloads != 2 {
+		t.Error("did not add MaximumActiveDownloads")
+	}
+	if len(cs.Config.Destinations) != 0 {
+		t.Error("incorrect number of destinations from migrated file")
+	}
+	if assert.Len(t, cs.Config.DownloadOptions, 2) {
+		if assert.Len(t, cs.Config.DownloadOptions[0].Args, 2) {
+			assert.Equal(t, "-o", cs.Config.DownloadOptions[0].Args[0])
+			assert.Equal(t, "/home/path/somegifs/%(title)s [%(id)s].%(ext)s", cs.Config.DownloadOptions[0].Args[1])
+			assert.Equal(t, "-o", cs.Config.DownloadOptions[1].Args[0])
+			assert.Equal(t, "/home/path/otherstuff/%(title)s [%(id)s].%(ext)s", cs.Config.DownloadOptions[1].Args[1])
+		}
+	}
+	os.Remove(cs.ConfigPath)
+}
+
 func configServiceFromString(configString string) *ConfigService {
 	tmpFile, _ := os.CreateTemp("", "gropple_test_*.yml")
 	tmpFile.Write([]byte(configString))