delta: return valid enum member

If we can't figure out what endianness a delta is, we should just throw
ENDIAN_INVALID.

Resolves: #550

Closes: #553
Approved by: cgwalters

src/libostree/ostree-repo-static-delta-core.c

@@ -764,7 +764,7 @@ _ostree_delta_get_endianness (GVariant *superblock,
           }
       }
 
-    return G_BYTE_ORDER;
+    return OSTREE_DELTA_ENDIAN_INVALID;
   }
 }