JSON_STRING_INVALID returned for incomplete true, false, null

[vikethehube]

Add these to incomplete_tests in unit_test.c: "{a:t", "{a:f", "{a:n",
and you get a test failure- the parser returns JSON_STRING_INVALID when it should return JSON_STRING_INCOMPLETE.

This is because the tests in parse_value for true, false, and null attempt to match the entire value only. They need to match a partial value as well, and if it is partial, return JSON_STRING_INCOMPLETE.

May I suggest something like this (for each of null, true, and false):

} else if (ch == 'n') {
  TRY(test_and_skip(f, 'n'));
  TRY(test_and_skip(f, 'u'));
  TRY(test_and_skip(f, 'l'));
  TRY(test_and_skip(f, 'l'));
  TRY(capture_ptr(f, f->cur - 4, JSON_TYPE_NULL));
  capture_len(f, f->num_tokens - 1, f->cur);

Which seems to work, but is a little bit ugly.