s3:auth: Avoid passing freed pamh pointer to funcitons using it

s3:auth: Avoid passing freed pamh pointer to funcitons using it

"Error: USE_AFTER_FREE (CWE-416):
samba-4.20.0rc2/source3/auth/pampass.c:466: freed_arg: ""pam_end"" frees ""pamh"".
samba-4.20.0rc2/source3/auth/pampass.c:467: pass_freed_arg: Passing freed pointer ""pamh"" as an argument to ""smb_pam_error_handler"".
  465|   	if( pamh != NULL ) {
  466|   		pam_error = pam_end(pamh, 0);
  467|-> 		if(smb_pam_error_handler(pamh, pam_error, ""End Cleanup Failed"", 2) == True) {
  468|   			DEBUG(4, (""smb_pam_end: PAM: PAM_END OK.\n""));
  469|   			return True;"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Noel Power <npower@samba.org>