json: Clean up how lexer consumes "end of input"

json: Clean up how lexer consumes "end of input"

When the lexer isn't in its start state at the end of input, it's
working on a token.  To flush it out, it needs to transit to its start
state on "end of input" lookahead.

There are two ways to the start state, depending on the current state:

* If the lexer is in a TERMINAL(JSON_FOO) state, it can emit a
  JSON_FOO token.

* Else, it can go to IN_ERROR state, and emit a JSON_ERROR token.

There are complications, however:

* The transition to IN_ERROR state consumes the input character and
  adds it to the JSON_ERROR token.  The latter is inappropriate for
  the "end of input" character, so we suppress that.  See also recent
  commit a2ec6be72b8 "json: Fix lexer to include the bad character in
  JSON_ERROR token".

* The transition to a TERMINAL(JSON_FOO) state doesn't consume the
  input character.  In that case, the lexer normally loops until it is
  consumed.  We have to suppress that for the "end of input" input
  character.  If we didn't, the lexer would consume it by entering
  IN_ERROR state, emitting a bogus JSON_ERROR token.  We fixed that in
  commit bd3924a33a6.

However, simply breaking the loop this way assumes that the lexer
needs exactly one state transition to reach its start state.  That
assumption is correct now, but it's unclean, and I'll soon break it.
Clean up: instead of breaking the loop after one iteration, break it
after it reached the start state.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180831075841.13363-3-armbru@redhat.com>