Add some missing `VERIFY_CHECK(ctx != NULL)` for functions that use `ARG_CHECK`
Add some missing `VERIFY_CHECK(ctx != NULL)` for functions that use `ARG_CHECK`

The `ARG_CHECK` macro requires that a variable called `ctx` exist and be
non-NULL. However, in several functions that do not use the context variable,
we simply ignore it with `(void)ctx`. Replace these with explicit checks for
non-NULLness to avoid invalid memory accesses.

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