MINOR: shctx: Set last_append to NULL when reserving block in hot list

Ensure that the last_append pointer is always set to NULL on first block
of rows reserved by the subsystems using the shctx (cache for instance).
This pointer will be used directly in shctx_row_data_append instead of
the 'from' param which will simplify its uses.

src/shctx.c

@@ -114,6 +114,7 @@ struct shared_block *shctx_row_reserve_hot(struct shared_context *shctx,
 					ret->block_count = freed;
 					ret->refcount = 1;
 					ret->last_reserved = block;
+					ret->last_append = NULL;
 					enough = 1;
 					break;
 				}