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 <jacob.champion@enterprisedb.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAOYmi+n9QWr4gsAADZc6qFQjFViXQYVk=gBy_EvxuqsgPJcb_g@mail.gmail.com

src/common/jsonapi.c

@@ -1536,7 +1536,7 @@ json_lex(JsonLexContext *lex)
 		jsonapi_StrValType *ptok = &(lex->inc_state->partial_token);
 		size_t		added = 0;
 		bool		tok_done = false;
-		JsonLexContext dummy_lex;
+		JsonLexContext dummy_lex = {0};
 		JsonParseErrorType partial_result;
 
 		if (ptok->data[0] == '"')