remove additional zero byte when writing pub der

Remove `- 1` for setting location of output buffer,
which added a leading zero which cause failure in ASN1 parsing.
Fixes #1257

programs/pkey/key_app_writer.c

@@ -128,7 +128,7 @@ static int write_public_key( mbedtls_pk_context *key, const char *output_file )
             return( ret );
 
         len = ret;
-        c = output_buf + sizeof(output_buf) - len - 1;
+        c = output_buf + sizeof(output_buf) - len;
     }
 
     if( ( f = fopen( output_file, "w" ) ) == NULL )