jsonapi: fully initialize dummy lexer

jsonapi: fully initialize dummy lexer

Valgrind reports that checks on lex->inc_state are undefined for the
"dummy lexer" used for incremental parsing, since it's only partially
initialized on the stack. This was introduced in 0785d1b8b2.
Zero-initialize the whole struct.

Author: Jacob Champion <[email protected]>
Reported-by: Alexander Lakhin <[email protected]>
Discussion: https://www.postgresql.org/message-id/CAOYmi+n9QWr4gsAADZc6qFQjFViXQYVk=gBy_EvxuqsgPJcb_g@mail.gmail.com