Fix: %mem-set OFFSET has no default, unlike %mem-ref

[dg1sbg]

What is broken

The methods generated for %mem-set in fli.lisp take &optional offset with no default, while the matching %mem-ref methods take &optional (offset 0). Calling %mem-set with the documented three arguments therefore passes NIL through to %offset-address-as-integer and fails:

(defparameter *p* (clasp-ffi:%allocate-foreign-object :float))

(clasp-ffi:%mem-ref *p* :float)         ; => 0.0     ok
(clasp-ffi:%mem-set *p* :float 2.5)     ; => error:  NIL is not of type INTEGER.
(clasp-ffi:%mem-set *p* :float 2.5 0)   ; => ok

So an offset must be supplied explicitly to %mem-set but not to %mem-ref. Both generic functions declare &optional offset, so the two accessors were plainly meant to be symmetric — only the generated methods disagree.

Fix

One token in the generate-methods macrolet: &optional offset becomes &optional (offset 0), matching the %mem-ref method generated three lines above it.

Compatibility

Strictly additive. The only arity whose behaviour changes is the 3-argument call, which previously errored unconditionally:

• 3 args — was an error, now works as offset 0.
• 4 args with an offset — unchanged.
• 4 args with an explicit NIL — unchanged, still errors, because an explicitly supplied argument means the default form never runs.

No call that worked before behaves differently.

Tests

Adds mem-set-default-offset to the FFI suite in defcallback-native.lisp. The pre-existing cffi-defcallback test exercises %mem-set with explicit offsets, so the 4-argument path is covered by an existing test rather than only by argument.

Full regression suite on boehm: 1975 successes, zero unexpected failures (1974 baseline + the new test). Passed MEM-SET-DEFAULT-OFFSET, Passed CFFI-DEFCALLBACK.

Found while investigating #1811, but entirely independent of it — this is a plain lambda-list oversight, not related to dispatch.