pull up r20551 from trunk

pull up r20551 from trunk

 r20551@cathode-dark-space:  jaltman | 2008-07-21 13:44:43 -0400
 ticket: 5080
 tags: pullup

 ccdefault.c:
 krb5_cc_default_name() is permitted to return a NULL
 pointer as a valid output.  Passing a NULL pointer to
 strcmp() will result in an exception as NULL is not
 a valid input parameter to strcmp().

 Save the output of krb5_cc_default_name() to a variable
 and modify the conditional to set the new default ccache
 name in the case where there is no existing default
 ccache name.

ticket: 5980

git-svn-id: svn://anonsvn.mit.edu/krb5/branches/krb5-1-6@20564 dc483132-0cff-0310-8789-dd5450dbe970