Description of problem:
While working on bug 679848, we discovered a new problem: Running yppasswdd with the -x option to pass data to an external program to update the source files and maps may insert garbage characters in front of the username.

It's supposed to pass the data to the external program on stdin in the format
  username o:oldpassword p:newpassword s:shell g:gecos\n

In my testing, 'homer' sometimes comes into the external program as 'pauhomer' or other garbage prefixed to homer.

Looking at the source, I see it doesn't clear memory after malloc; from
ypserv-2.19/rpc.yppasswdd/update.c:
   873    if ((parentmsg = malloc (msglen)) == NULL)
   874      {
   875        log_msg ("rpc.yppasswdd: out of memory");
   876        return res;
   877      }
   878
   879    strcat (parentmsg, yppw->newpw.pw_name);

Inserting a memset() call before line 879 fixes the problem.

Version-Release number of selected component (if applicable):
ypserv-2.19-5

How reproducible:
almost always

Steps to Reproduce:
1. modify /etc/sysconfig/yppasswdd and set
   YPPASSWDD_ARGS="-x /usr/lib/yp/external-test"
2. create /usr/lib/yp/external-test with contents
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#!/usr/bin/perl
$data = <>;
open DEBUG, ">/tmp/external-test.log";
print DEBUG "$data";
close DEBUG;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3. service rpc.yppasswd start
4. on a NIS client, run yppasswd to change homer's password
5. watch /tmp/external-test.log on the NIS server
  
Actual results:
pauhomer o:password p:5FsD7UxMQ5mnU

Expected results:
homer o:password p:5FsD7UxMQ5mnU

Additional info: