Issue #876: Fix integer overflow on slice_count

If the slice count doesn't fit into uint32_t, consider that the
memory allocation failed.

On s390x, allocating around 8,589,934,592 GiB with mmap() works
thanks to overcommit on a machine with 8 GiB of memory:

    mmap(NULL, 0x8000000000400000,
         PROT_READ|PROT_WRITE,
         MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE,
         -1, 0)

Author: vstinner

src/segment.c

@@ -820,6 +820,9 @@ static mi_segment_t* mi_segment_os_alloc( size_t required, size_t page_alignment
     const size_t extra = align_offset - info_size;
     // recalculate due to potential guard pages
     *psegment_slices = mi_segment_calculate_slices(required + extra, ppre_size, pinfo_slices);
+
+    // gh-876: mi_page_t.slice_count type is uint32_t
+    if (*psegment_slices > (size_t)UINT32_MAX) return NULL;
   }
 
   const size_t segment_size = (*psegment_slices) * MI_SEGMENT_SLICE_SIZE;
@@ -870,6 +873,9 @@ static mi_segment_t* mi_segment_alloc(size_t required, size_t page_alignment, mi
   size_t pre_size;
   size_t segment_slices = mi_segment_calculate_slices(required, &pre_size, &info_slices);
 
+  // gh-876: mi_page_t.slice_count type is uint32_t
+  if (segment_slices > (size_t)UINT32_MAX) return NULL;
+
   // Commit eagerly only if not the first N lazy segments (to reduce impact of many threads that allocate just a little)
   const bool eager_delay = (// !_mi_os_has_overcommit() &&             // never delay on overcommit systems
                             _mi_current_thread_count() > 1 &&       // do not delay for the first N threads