fix jl_gc_realloc_string size miscomputation

fix JuliaLang#22021

src/gc.c

@@ -2792,7 +2792,8 @@ jl_value_t *jl_gc_realloc_string(jl_value_t *s, size_t sz)
  return snew;
  }
  size_t newsz = sz + sizeof(size_t) + 1;
- size_t offs = offsetof(bigval_t, header);
+ size_t offs = sizeof(bigval_t);
+ size_t oldsz = LLT_ALIGN(strsz + offs, JL_CACHE_BYTE_ALIGNMENT);
  size_t allocsz = LLT_ALIGN(newsz + offs, JL_CACHE_BYTE_ALIGNMENT);
  if (allocsz < sz) // overflow in adding offs, size was "negative"
  jl_throw(jl_memory_exception);
@@ -2804,9 +2805,7 @@ jl_value_t *jl_gc_realloc_string(jl_value_t *s, size_t sz)
  // the old pointer to be left alone if we can't grow in place.
  // for now it's up to the caller to make sure there are no references to the
  // old pointer.
- bigval_t *newbig =
- (bigval_t*)gc_managed_realloc_(ptls, hdr, allocsz, LLT_ALIGN(strsz+offs, JL_CACHE_BYTE_ALIGNMENT),
- 1, s, 0);
+ bigval_t *newbig = (bigval_t*)gc_managed_realloc_(ptls, hdr, allocsz, oldsz, 1, s, 0);
  newbig->sz = allocsz;
  newbig->age = 0;
  gc_big_object_link(newbig, &ptls->heap.big_objects);