Since we already successfully parsed the attribute name and other
items, it is guaranteed that we made progress in the input stream.

Comparing the input pointer to a previous value also looks fragile to
me. What if the input buffer was reallocated and the new "cur" pointer
happens to be the same as the old one? There are a couple of similar
checks which also take "consumed" into account. This seems to be safer
but I'm not convinced that it couldn't lead to false alarms in rare
situations.