girffi.c: fix return value for g_callable_info_prepare_closure()

The initial failure was observed on meld against recently released libffi-3.4-rc1. There meld crashes as:

$ meld
Segmentation fault (core dumped)

$ gdb --args /usr/bin/python3.9 /usr/bin/meld
(gdb) run
...
Thread 1 "python3.9" received signal SIGSEGV, Segmentation fault.
0x00007fffe9ac1ae8 in g_callable_info_free_closure (
  callable_info=0x555555d45990, closure=0x7fffe9e70c20)
    at ../gobject-introspection-1.68.0/girepository/girffi.c:428
428       g_free (wrapper->ffi_closure.cif->arg_types);
(gdb) bt
  callable_info=0x555555d45990, closure=0x7fffe9e70c20)
    at ../gobject-introspection-1.68.0/girepository/girffi.c:428
  data=0x555555d252d0)
    at ../pygobject-3.40.1/gi/pygi-closure.c:635
...

The bug here is in type mismatch between expected return value of g_callable_info_prepare_closure() and actual value (executable code pointer):

ffi_closure * g_callable_info_prepare_closure(...) {
    gpointer exec_ptr;
    ...
    status = ffi_prep_closure_loc (&closure->ffi_closure, cif, callback, user_data, exec_ptr);

    return exec_ptr;
}

Note: exec_ptr is a code pointer that could be directly executed by caller, like ((rt (*)(a1,a2))exec_ptr)(1,2); It should never be wrapped into an ffi_closure*, which is normally called via ffi_call(closure, ...).

We see the problem when we try to free direct code pointer instead of ffi_closure() as starting from libffi-3.4 executable trampoline and ffi_closure() don't necessarily live in the same block:

https://github.com/libffi/libffi/commit/9ba559217bea0803263a9a9a0bafcf9203606f5b

Signed-off-by: Sergei Trofimovich slyfox@gentoo.org