fix bug caught by test suite

Types/Key.hs

@@ -344,7 +344,7 @@ parseKeyVariety "URL"          = URLKey
 parseKeyVariety b
 	| "X" `S.isPrefixOf` b = 
 		let b' = S.tail b
-		in if S.last b' == fromIntegral (ord 'E')
+		in if not (S.null b') && S.last b' == fromIntegral (ord 'E')
 			then ExternalKey (S.init b') (HasExt True)
 			else ExternalKey b' (HasExt False)
 	| otherwise = OtherKey b

doc/bugs/prop__95__isomorphic__95__key__95__encode_fails_with_external_backend_changes.mdwn

@@ -33,3 +33,5 @@ Thanks in advance.
 
 [[!meta author=kyle]]
 [[!tag projects/datalad]]
+
+> [[fixed|done]] and rest of test suite passing --[[Joey]]