fix(vm): validate arg count before OpCall

Prevent index out of bounds panic when calling a function with more
arguments than it accepts. The VM now validates argument count before
attempting to access parameter types, providing a clear error message
instead of a cryptic "index out of range [0] with length 0" panic.

This issue was discovered by clusterfuzz with the expression:
$env(''matches'  '? :now().UTC(g).d)//

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

Author: thevilledev

vm/vm.go

@@ -372,9 +372,18 @@ func (vm *VM) Run(program *Program, env any) (_ any, err error) {
 			}
 			fnType := fn.Type()
 			size := arg
-			in := make([]reflect.Value, size)
 			isVariadic := fnType.IsVariadic()
 			numIn := fnType.NumIn()
+			if isVariadic {
+				if size < numIn-1 {
+					panic(fmt.Sprintf("invalid number of arguments: expected at least %d, got %d", numIn-1, size))
+				}
+			} else {
+				if size != numIn {
+					panic(fmt.Sprintf("invalid number of arguments: expected %d, got %d", numIn, size))
+				}
+			}
+			in := make([]reflect.Value, size)
 			for i := int(size) - 1; i >= 0; i-- {
 				param := vm.pop()
 				if param == nil {

vm/vm_test.go

@@ -1491,3 +1491,22 @@ func TestVM_StackUnderflow(t *testing.T) {
 		})
 	}
 }
+
+func TestVM_OpCall_InvalidNumberOfArguments(t *testing.T) {
+	// This test ensures that calling a function with wrong number of arguments
+	// produces a clear error message instead of a panic.
+	// Regression test for clusterfuzz issue with expression:
+	// $env(''matches'  '? :now().UTC(g).d)//
+
+	env := map[string]any{
+		"ok": true,
+	}
+
+	code := `$env('' matches ' '? : now().UTC(g))`
+	program, err := expr.Compile(code, expr.Env(env))
+	require.NoError(t, err)
+
+	_, err = expr.Run(program, env)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "invalid number of arguments")
+}