heap: Only fill new heaps with FREE_FILL_PATTERN if Comprehensive poisoning is on

Significantly speeds up heap initialization at startup when default "Light" heap
poisoning is enabled.

Tip via reddit user LinkeSeitentasche https://www.reddit.com/r/esp32/comments/fnj51a/a_guide_to_improving_esp32_boot_speed/

components/heap/multi_heap_poisoning.c

@@ -355,9 +355,11 @@ void *multi_heap_get_block_owner(multi_heap_block_handle_t block)
 
 multi_heap_handle_t multi_heap_register(void *start, size_t size)
 {
+#ifdef SLOW
     if (start != NULL) {
         memset(start, FREE_FILL_PATTERN, size);
     }
+#endif
     return multi_heap_register_impl(start, size);
 }