Handle NULL returns from glibc 2.17+ crypt().

Starting with glibc 2.17 (eglibc 2.17), crypt() fails with EINVAL
(w/ NULL return) if the salt violates specifications. Additionally,
on FIPS-140 enabled Linux systems, DES/MD5-encrypted passwords
passed to crypt() fail with EPERM (w/ NULL return).

If using glibc's crypt(), check return value to avoid a possible
NULL pointer dereference.

programs/pluto/xauth.c

@@ -1070,8 +1070,8 @@ int do_md5_authentication(void *varg)
 		openswan_log("XAUTH: checking user(%s:%s) " , szuser, szconnid);
 	    }
 
-           /* Ok then now password check */
-           if ( strcmp(cp, szpass ) == 0 )
+           /* Ok then now password check; Note: handle crypt() NULL returns */
+           if ( cp && strcmp(cp, szpass ) == 0 )
            {
              /* we have a winner */
              fclose( fp );