pull up r20555 from trunk

pull up r20555 from trunk

 r20555@cathode-dark-space:  jaltman | 2008-07-21 15:43:21 -0400
 ticket: 5895
 tags: pullup

 There are two mutex locking issues that Roland Dowdeswell noticed in
 the memory ccache.  The first one is in cc_memory.c:krb5_mcc_initialize().
 When it is free(3)ing the existing credentials it does not lock the
 data structures and hence two separate threads can run into issues.

 The same problem exists in cc_memory.c:krb5_mcc_destroy().

ticket: 5895
status: resolved
version_fixed: 1.6.4

git-svn-id: svn://anonsvn.mit.edu/krb5/branches/krb5-1-6@20562 dc483132-0cff-0310-8789-dd5450dbe970