Fix to cvmfs_listxattr

The listxattr fuse function (cvmfs_listxattr) returns a list that contains an entry with a null string for regular files, which causes some xattr programs that don't check for this to error out when they subsequently do a getxattr on the empty string. This is because when the C++ std:string objects are constructed, the sizeof the char[] contains the terminating null in addition to the null explicitly within the string. 

This fix changes the two instances of sizeof(char[]) to be sizeof(char[])-1 to address this issue.