hivex: Don't die on valid registries which have bad declared data lengths.

hivex: Don't die on valid registries which have bad declared data lengths.

Some apparently valid registries contain value data length
declarations which exceed the allocated block size for the
value.

Previously the code would return EFAULT for such registries.
However since these appear to be otherwise valid registries,
turn this into a warning and just use the allocated block size
as the data length (in other words, truncate the value).