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.

src/blob.c

@@ -80,6 +80,7 @@ blob_copy(blob_T *from, typval_T *to)
 		len = 0;
 	}
 	to->vval.v_blob->bv_ga.ga_len = len;
+	to->vval.v_blob->bv_ga.ga_maxlen = len;
     }
     return ret;
 }

src/testdir/test_vim9_func.vim

@@ -1790,6 +1790,12 @@ def Test_list_add()
   CheckDefExecFailure(lines, 'E1130:', 2)
 enddef
 
+def Test_blob_add()
+  var b: blob = 0z12
+  add(b, 0x34)
+  assert_equal(0z1234, b)
+enddef
+
 def SID(): number
   return expand('<SID>')
           ->matchstr('<SNR>\zs\d\+\ze_$')

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1866,
 /**/
     1865,
 /**/