Initialize the key registers in the skip-scan optimization to NULL, in case

Initialize the key registers in the skip-scan optimization to NULL, in case
the query is run on a corrupt database that causes the initialization of the
key to be bypassed.
dbsqlfuzz 62fdf2bece00d24cac8a4edf2cf562e6eeac779d

FossilOrigin-Name: 8b54102a00852b72783b5b2f336c7a093adbdc8e1b94d46b7d315ae6e5d9476b