vim-patch:8.2.1866: Vim9: appending to pushed blob gives wrong result

Problem:    Vim9: appending to pushed blob gives wrong result.
Solution:   Set ga_maxlen when copying a blob.
66fa5fd54f

Vim9script is N/A.

src/nvim/eval/typval.c

@@ -2274,6 +2274,7 @@ void tv_blob_copy(typval_T *const from, typval_T *const to)
           = xmemdup(from->vval.v_blob->bv_ga.ga_data, (size_t)len);
     }
     to->vval.v_blob->bv_ga.ga_len = len;
+    to->vval.v_blob->bv_ga.ga_maxlen = len;
   }
 }