Start arena block doubling at initial block size.

If an initial block is provided, we should start our
block doubling at the size of the initial block, not 128.
This saves us from unnecessary overhead when we overflow
the initial block.

upb/upb.c

@@ -178,15 +178,14 @@ upb_arena *upb_arena_init(void *mem, size_t n, upb_alloc *alloc) {
   }
 
   a = UPB_PTR_AT(mem, n - sizeof(*a), upb_arena);
-  n -= sizeof(*a);
 
   a->head.alloc.func = &upb_arena_doalloc;
   a->block_alloc = alloc;
   a->parent = a;
   a->refcount = 1;
-  a->last_size = 128;
+  a->last_size = UPB_MAX(128, n);
   a->head.ptr = mem;
-  a->head.end = UPB_PTR_AT(mem, n, char);
+  a->head.end = UPB_PTR_AT(mem, n - sizeof(*a), char);
   a->freelist = NULL;
   a->cleanups = NULL;