patch 9.0.1662: crash when using a class member twice

Problem:    Crash when using a class member twice. (Christian J. Robinson)
Solution:   Make a copy of the value.

src/testdir/test_vim9_class.vim

@@ -838,6 +838,23 @@ def Test_class_member()
   END
   v9.CheckScriptSuccess(lines)
 
+  # using static class member twice
+  lines =<< trim END
+      vim9script
+
+      class HTML
+        static author: string = 'John Doe'
+
+        static def MacroSubstitute(s: string): string
+          return substitute(s, '{{author}}', author, 'gi')
+        enddef
+      endclass
+
+      assert_equal('some text', HTML.MacroSubstitute('some text'))
+      assert_equal('some text', HTML.MacroSubstitute('some text'))
+  END
+  v9.CheckScriptSuccess(lines)
+
   # access private member in lambda
   lines =<< trim END
       vim9script

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1662,
 /**/
     1661,
 /**/

src/vim9execute.c

@@ -3967,8 +3967,8 @@ exec_instructions(ectx_T *ectx)
 		    if (GA_GROW_FAILS(&ectx->ec_stack, 1))
 			goto theend;
 		    classmember_T *cm = &iptr->isn_arg.classmember;
-		    *STACK_TV_BOT(0) =
-				    cm->cm_class->class_members_tv[cm->cm_idx];
+		    copy_tv(cm->cm_class->class_members_tv + cm->cm_idx,
+							      STACK_TV_BOT(0));
 		    ++ectx->ec_stack.ga_len;
 		}
 		break;