mediumslab: limit header to min of page or slab

This had not been observed as an issue prior to
923705e514 because CMakeLists.txt had, until
then, been using EQUAL, not STREQUAL, to test for oe (and to then enable
USE_SMALL_CHUNKS).  This test would fail, and so the default SLAB_SIZE was
used.  Absent this min operation, the use of a whole page on a 64KiB page
causes a crash when using the largest medium size class, as, ultimately, size
classes are not based on page sizes, and so committing a whole page to the
header leaves too little room for that class.

See also 3d3b048776.

src/mem/allocconfig.h

@@ -100,7 +100,7 @@ namespace snmalloc
   static constexpr size_t MIN_ALLOC_SIZE = 2 * sizeof(void*);
   static constexpr size_t MIN_ALLOC_BITS = bits::ctz_const(MIN_ALLOC_SIZE);
 
-  // Slabs are 64 KiB unless constrained to 16 KiB.
+  // Slabs are 64 KiB unless constrained to 16 or even 8 KiB
   static constexpr size_t SLAB_BITS =
     USE_SMALL_CHUNKS ? 13 : (USE_LARGE_CHUNKS ? 16 : 14);
   static constexpr size_t SLAB_SIZE = 1 << SLAB_BITS;