Bug 235647

The pam_verror may cause segmentation fault.

Consider the following scenario:

1. A service module (for instance, pam_unix) calls PAM_VERBOSE_ERROR.
   That macro expands to _pam_verbose_error call.

2. _pam_verbose_error calls pam_verror if the PAM_SILENT flag is not 
  set on the PAM handle and no_warn option is not set for the service 
  module.

3. pam_verror allocates 'char *rsp' on stack w/o initializing it to NULL,
   (a dangling pointer), and makes 
   pam_vprompt(pamh, PAM_ERROR_MSG, &rsp, fmt, ap) call. 

4. Now if the the PAM conversation is NULL, pam_vprompt soon returns 
   w/ PAM_SYSTEM_ERR and __does not__ set *rsp. 

5. pam_verror then does FREE(rsp) and if rsp happens to be anything other 
   than NULL, a segmentation fault happens.