squidpurge: fix META TLV parsing issues

squidpurge: fix META TLV parsing issues

* current Squid may stoe objects with up to 64KB URLs and 64KB headers
  in the disk cache. Increas the tool buffer to 128KB to prevent
  truncating the loaded meta data.

* check for and report when meta data overruns the end of loaded buffer
  content. Ignore the truncated TLV entry and produce a WARNING.

* validate the TLV size field loaded from disk to prevent buffer overrun
  errors from corrupted files on disk.

 Detected by Coverity Scan. Issues 740381, 740488.