fixup! mimalloc: use "weak" random seed when statically linked

Always use the internal "use_weak" random seed when initializing
the "mimalloc" heap when statically linked on Windows.

The imported "mimalloc" routines support several random sources
to seed the heap data structures, including BCrypt.dll and
RtlGenRandom.  Crashes have been reported when using BCrypt.dll
if it initialized during an `atexit()` handler function.  Granted,
such DLL initialization should not happen in an atexit handler,
but yet the crashes remain.

It should be noted that on Windows when statically linked, the
mimalloc startup code (called by the GCC CRT to initialize static
data prior to calling `main()`) always uses the internal "weak"
random seed.  "mimalloc" does not try to load an alternate
random source until after the OS initialization has completed.

Heap data is stored in `__declspec(thread)` TLS data and in theory
each Git thread will have its own heap data.  However, testing
shows that the "mimalloc" library doesn't actually call
`os_random_buf()` (to load a new random source) when creating these
new per-thread heap structures.

However, if an atexit handler is forced to run on a non-main
thread, the "mimalloc" library *WILL* try to create a new heap
and seed it with `os_random_buf()`.  (The reason for this is still
a mystery to this author.)  The `os_random_buf()` call can cause
the (previously uninitialized BCrypt.dll library) to be dynamically
loaded and a call made into it.  Crashes have been reported in
v2.40.1.vfs.0.0 while in this call.

As a workaround, the fix here forces the use of the internal
"use_weak" random code for the subsequent `os_random_buf()` calls.
Since we have been using that random generator for the majority
of the program, it seems safe to use it for the final few mallocs
in the atexit handler (of which there really shouldn't be that many.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mimalloc/init.c

@@ -302,7 +302,11 @@ static bool _mi_heap_init(void) {
     _mi_memcpy_aligned(tld, &tld_empty, sizeof(*tld));
     _mi_memcpy_aligned(heap, &_mi_heap_empty, sizeof(*heap));
     heap->thread_id = _mi_thread_id();
+    #if defined(_WIN32) && !defined(MI_SHARED_LIB)
+    _mi_random_init_weak(&heap->random); // match mi_heap_main_init()
+    #else
     _mi_random_init(&heap->random);
+    #endif
     heap->cookie  = _mi_heap_random_next(heap) | 1;
     heap->keys[0] = _mi_heap_random_next(heap);
     heap->keys[1] = _mi_heap_random_next(heap);