* common-src/amflock.c: Detect failure to write the file and ftruncate
  it to 0.
* perl/Amanda/Changer.pm: Detect failure to read the state file and do
  like if it was empty.


git-svn-id: https://svn.code.sf.net/p/amanda/code/amanda/trunk@5627 a8d146d6-cc15-0410-8900-af154a0219e0