Return real length of buffer from hivex_value_value.

Return real length of buffer from hivex_value_value.

In real registries, often the length declared in the header does not
match the length of the block.  In this case hivex_value_value would
only allocate a value with a size which is the shorter of the two
length values, which is correct and safe.

However user code could do:

  buf = hivex_value_value (h, v, &t, &len);
  memcpy (somewhere, buf, len);

which would copy uninitialized data.

If hivex_value_value truncates a value like this, we also need to
return the shorter length to the user as well.