[crypto] Free correct pointer on the error path

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/pem.c

 	*cursor = malloc ( sizeof ( **cursor ) + decoded_max_len );
 	if ( ! *cursor ) {
 		rc = -ENOMEM;
-		goto err_alloc_decoded;
+		goto err_alloc_cursor;
 	}
 	decoded = ( ( ( void * ) *cursor ) + sizeof ( **cursor ) );
 
 	return offset;
 
  err_decode:
-	free ( decoded );
- err_alloc_decoded:
+	free ( *cursor );
+	*cursor = NULL;
+ err_alloc_cursor:
 	free ( encoded );
  err_alloc_encoded:
  err_end: