patch 9.2.1222: Vim9: Stack corruption with :silent! causes a crash

Problem:  Vim9: when a function called with :silent! fails to load
          a variable, the value is missing from the stack causing a
          crash (waroad).
Solution: Push a zero value.

closes: #21519

Assisted-by: Claude
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_func.vim

@@ -5141,4 +5141,32 @@ def Test_lambda_type_error()
   v9.CheckSourceFailure(lines, 'E1180: Variable arguments type must be a list: number', 2)
 enddef
 
+def Test_cmdmod_silent_nested_load_error()
+  var lines =<< trim END
+      vim9script
+      var result = ''
+
+      def H(): number
+        result ..= 'Hb'
+        echo g:nosuch
+        result ..= 'Ha'
+        return 1
+      enddef
+
+      def G()
+        result ..= 'Gb'
+        silent! H()
+        silent! g:out->add(H())
+        result ..= 'Ga'
+      enddef
+
+      g:out = []
+      G()
+      assert_equal('GbHbHaHbHaGa', result)
+      assert_equal([1], g:out)
+      unlet g:out
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -743,6 +743,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1222,
 /**/
     1221,
 /**/

src/vim9execute.c

@@ -3306,6 +3306,13 @@ load_namespace_var(ectx_T *ectx, isntype_T isn_type, isn_T *iptr)
 	else
 	    semsg(_(e_undefined_variable_char_str),
 					      namespace, iptr->isn_arg.string);
+	// Push a value anyway, the stack must have the expected size when the
+	// error is ignored because of ":silent!" in a calling function.
+	typval_T *tv = STACK_TV_BOT(0);
+	tv->v_type = VAR_NUMBER;
+	tv->v_lock = 0;
+	tv->vval.v_number = 0;
+	++ectx->ec_stack.ga_len;
 	return FAIL;
     }
     else
@@ -4268,7 +4275,14 @@ exec_instructions(ectx_T *ectx)
 		    SOURCING_LNUM = iptr->isn_lnum;
 		    if (eval_variable(name, (int)STRLEN(name), 0,
 			      STACK_TV_BOT(0), NULL, EVAL_VAR_VERBOSE) == FAIL)
+		    {
+			tv = STACK_TV_BOT(0);
+			tv->v_type = VAR_NUMBER;
+			tv->v_lock = 0;
+			tv->vval.v_number = 0;
+			++ectx->ec_stack.ga_len;
 			goto on_error;
+		    }
 		    ++ectx->ec_stack.ga_len;
 		}
 		break;