Runtime exception when storing network server login data for invalid URLs

VLCNetworkServerLoginInformation+Keychain.m:42 keychainServiceIdentifier can return nil, which will lead to a runtime exception as the keychain service object may not be nil.

1   libobjc.A.dylib               	0x19541fc00 objc_exception_throw + 60 (objc-exception.mm:356)
2   CoreFoundation                	0x19cfac568 0x19cf93000 + 103784
3   VLC for iOS                   	0x1010cf728 +[XKKeychainGenericPasswordItem queryDictionaryForService:account:] + 160 (XKKeychainGenericPasswordItem.m:169)
4   VLC for iOS                   	0x1010cea00 +[XKKeychainGenericPasswordItem itemForService:account:error:] + 48 (XKKeychainGenericPasswordItem.m:17)
5   VLC for iOS                   	0x100a75e10 -[VLCNetworkServerLoginInformation(Keychain) saveLoginInformationToKeychainWithError:] + 152 (VLCNetworkServerLoginInformation+Keychain.m:75)
6   VLC for iOS                   	0x100a28238 -[VLCNetworkLoginDataSourceSavedLogins saveLogin:error:] + 68 (VLCNetworkLoginDataSourceSavedLogins.m:109)
7   VLC for iOS                   	0x100a51164 -[VLCNetworkLoginViewController saveLoginDataSource:] + 404 (VLCNetworkLoginViewController.m:204)
8   VLC for iOS                   	0x100a656a4 -[VLCNetworkLoginDataSourceLogin didSelectRow:] + 164 (VLCNetworkLoginDataSourceLogin.m:225)
9   VLC for iOS                   	0x100a16d1c -[VLCNetworkLoginDataSource tableView:didSelectRowAtIndexPath:] + 180 (VLCNetworkLoginDataSource.m:121)