jpeg reading: fix files with broken chunk lengths by reading until next 0xFF marker

  If such a chunk is encountered, then a "Warn" entry is pushed into
  the info result.
new test jpeg with a corrupt (too long) chunk

Also added new test script t/jpg_corruptchunk.t and
amended string.t because of the additional warning.
* preparing 1.33_51
Merge branch 'jpeg-ff-marker'