scan-build is worried that `node->data->common.value_table->value_init`
will be a `NULL` pointer dereference in the assignment to
`node->mutatable_check_cache`.

There’s already an assertion immediately below to check against this, so
let’s move it up a line to help the static analyser out.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1767