text/template: avoid index-out-of-range panic when accessing args[0]

Fixes golang#70341

Change-Id: I3df0175929b4aed76522ef36aecfa924f3883d9e

src/text/template/exec.go

@@ -855,7 +855,7 @@ func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node
 
 	// Special case for the "call" builtin.
 	// Insert the name of the callee function as the first argument.
-	if isBuiltin && name == "call" {
+	if len(args) > 0 && isBuiltin && name == "call" {
 		calleeName := args[0].String()
 		argv = append([]reflect.Value{reflect.ValueOf(calleeName)}, argv...)
 		fun = reflect.ValueOf(call)

src/text/template/exec_test.go

@@ -1779,12 +1779,19 @@ func TestFunctionCheckDuringCall(t *testing.T) {
 		input   string
 		data    any
 		wantErr string
-	}{{
-		name:    "call nothing",
-		input:   `{{call}}`,
-		data:    tVal,
-		wantErr: "wrong number of args for call: want at least 1 got 0",
-	},
+	}{
+		{
+			name:    "call with no arguments",
+			input:   `{{ 1 | call }}`,
+			data:    tVal,
+			wantErr: "error calling call: unreachable",
+		},
+		{
+			name:    "call nothing",
+			input:   `{{call}}`,
+			data:    tVal,
+			wantErr: "wrong number of args for call: want at least 1 got 0",
+		},
 		{
 			name:    "call non-function",
 			input:   "{{call .True}}",