Use offsetof() when sizing dynamic structures.

Base dynamic structure size on offsetof(), rather than subtracting the
size of the dynamic structure member.  Results could differ on systems
with strict data structure alignment requirements.

jemalloc/src/jemalloc.c

@@ -101,8 +101,8 @@ arenas_extend(unsigned ind)
 	arena_t *ret;
 
 	/* Allocate enough space for trailing bins. */
-	ret = (arena_t *)base_alloc(sizeof(arena_t)
-	    + (sizeof(arena_bin_t) * (nbins - 1)));
+	ret = (arena_t *)base_alloc(offsetof(arena_t, bins)
+	    + (sizeof(arena_bin_t) * nbins));
 	if (ret != NULL && arena_new(ret, ind) == false) {
 		arenas[ind] = ret;
 		return (ret);