[NFC] Refactor initialisation of Darwin malloc zone fields into separate function called `InitMallocZoneFields()` which is now called from `ReplaceSystemMalloc()`.

[delcypher]

Summary

The motivation here is to be able support initialisation of the
malloc zone on other code paths.

rdar://problem/45284065

[delcypher]

@kubamracek Thanks for the quick review.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_malloc_mac.inc

boolean_t mi_zone_locked(malloc_zone_t *zone) {
   // UNIMPLEMENTED();
   return false;
 }
 
 }  // unnamed namespace
 
 namespace COMMON_MALLOC_NAMESPACE {
 
-void ReplaceSystemMalloc() {
+void InitMallocZoneFields() {
   static malloc_introspection_t sanitizer_zone_introspection;
   // Ok to use internal_memset, these places are not performance-critical.
   internal_memset(&sanitizer_zone_introspection, 0,
                   sizeof(sanitizer_zone_introspection));
 
   sanitizer_zone_introspection.enumerator = &mi_enumerator;
   sanitizer_zone_introspection.good_size = &mi_good_size;
   sanitizer_zone_introspection.check = &mi_check;
void InitMallocZoneFields() {
   sanitizer_zone.free = &__sanitizer_mz_free;
   sanitizer_zone.realloc = &__sanitizer_mz_realloc;
   sanitizer_zone.destroy = &__sanitizer_mz_destroy;
   sanitizer_zone.batch_malloc = 0;
   sanitizer_zone.batch_free = 0;
   sanitizer_zone.free_definite_size = 0;
   sanitizer_zone.memalign = &__sanitizer_mz_memalign;
   sanitizer_zone.introspect = &sanitizer_zone_introspection;
+}
+
+void ReplaceSystemMalloc() {
+  InitMallocZoneFields();
 
   // Register the zone.
   malloc_zone_register(&sanitizer_zone);
 }
 
 }  // namespace COMMON_MALLOC_NAMESPACE