parser: handle OOM gracefully

parser: handle OOM gracefully

Before, a failed malloc or strdup would lead to a NULL dereference.
* qparser.y (invalid): New file-scoped global.
(make_number): Never return NULL.  Instead, return &invalid.
(make_string): Likewise.
(make_tree): If an input has type T_INVALID, return it right away.
Upon allocation failure, return &invalid.
(make_comp, make_link): Likewise.
(make_link, make_string): Avoid unnecessary strdup, now that
lexed tokens are malloc'd.
Also, add some per-function comments.