json: Correct position tracking in JSON parser implementations.

json: Correct position tracking in JSON parser implementations.

When json_lex_input() returns false, the parser does not consume the byte
passed in.  That byte will get processed again in the next iteration of
the json_parser_feed() loop.  Therefore, until now, this code has
double-counted bytes that cause a false return from json_lex_input().

This fixes the problem.  Every input byte is now counted only once.

Signed-off-by: Ben Pfaff <blp@nicira.com>