fix oob tree index in parseClangAstDump on stray NULL node

[Nussu06]

parseClangAstDump indexes tree[level - 1] for a <<<NULL>>> line without the level == 0 || level > tree.size() check that the sibling path for normal nodes applies a few lines below, so a dump whose stray null child sits at the top indentation makes level zero and reads tree[-1]. Feeding a crafted or truncated clang AST dump through --clang trips a heap-buffer-overflow (confirmed under ASAN) in that loop. This adds the same guard the normal-node path already uses, plus a regression test next to the existing crash() case.