Update cmark-upstream to github/cmark-gfm@57d5e09

Author: phillmv

ext/commonmarker/arena.c

@@ -68,15 +68,16 @@ static void *arena_calloc(size_t nmem, size_t size) {
   const size_t align = sizeof(size_t) - 1;
   sz = (sz + align) & ~align;
 
+  struct arena_chunk *chunk;
   if (sz > A->sz) {
-    A->prev = alloc_arena_chunk(sz, A->prev);
-    return (uint8_t *) A->prev->ptr + sizeof(size_t);
+    A->prev = chunk = alloc_arena_chunk(sz, A->prev);
+  } else if (sz > A->sz - A->used) {
+    A = chunk = alloc_arena_chunk(A->sz + A->sz / 2, A);
+  } else {
+    chunk = A;
   }
-  if (sz > A->sz - A->used) {
-    A = alloc_arena_chunk(A->sz + A->sz / 2, A);
-  }
-  void *ptr = (uint8_t *) A->ptr + A->used;
-  A->used += sz;
+  void *ptr = (uint8_t *) chunk->ptr + chunk->used;
+  chunk->used += sz;
   *((size_t *) ptr) = sz - sizeof(size_t);
   return (uint8_t *) ptr + sizeof(size_t);
 }