[3.14] gh-140471: Fix buffer overflow in AST node initialization with malformed `_fields` (GH-140506) (#140509)

gh-140471: Fix buffer overflow in AST node initialization with malformed `_fields` (GH-140506)
(cherry picked from commit 95953b692db6cbd88139de12d81fb123293ec2d5)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>