Align pool allocator with 8 byte in 64-bit environment.

To avoid warnings from undefined sanitizer.
( https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html )

src/pool.c

@@ -13,8 +13,12 @@
 /* allocated memory address should be multiple of POOL_ALIGNMENT */
 /* or undef it if alignment does not matter */
 #ifndef POOL_ALIGNMENT
+#if INTPTR_MAX == INT64_MAX
+#define POOL_ALIGNMENT 8
+#else
 #define POOL_ALIGNMENT 4
 #endif
+#endif
 /* page size of memory pool */
 #ifndef POOL_PAGE_SIZE
 #define POOL_PAGE_SIZE 16000