Fix dangling pointer problem in ReorderBufferSerializeChange.
Commit
3fe3511d05127cc024b221040db2eeb352e7d716 introduced a new
case into this function, but neglected to ensure that the "ondisk"
pointer got updated after a possible reallocation as the code does
in other cases.
Stas Kelvich, per diagnosis by Konstantin Knizhnik.