In summary dumping a `catch(...)` statement using
IgnoreUnlessSpelledInSource AST traversal causes a seg fault, as the
variable declaration of the catch is `nullptr`.
Diagnosed the cause by attaching the debugger to `clang-query`, this PR
adds a fix to check for `nullptr` before accessing the `isImplicit()`
method of the `Decl` pointee in the AST node traverser visitor
Fixes#146101
44ec3e8f9c