lightningd: parent watchman's formatted blockhash strings to tmpctx

json_add_string copies its value, so the strings fmt_bitcoin_blkid
allocates in json_block_processed and json_getwatchmanheight are
referenced by nothing once the call returns.  They are parented to
the response stream and freed with it, but the memleak scanner works
by searching memory for pointers to each allocation: when a dev
memleak check races an in-flight response, the unreferenced string is
reported as a leak and fails the test run (seen in a liquid CI run of
test_bwatch_add_watch_creates_datastore_entry).  Parent them to
tmpctx, the idiom used elsewhere.

Fixes: #9362
Changelog-None

lightningd/watchman.c

@@ -755,7 +755,7 @@ static struct command_result *json_block_processed(struct command *cmd,
 	json_add_u32(response, "blockheight", *blockheight);
 	if (wm->last_processed_height > 0)
 		json_add_string(response, "blockhash",
-				fmt_bitcoin_blkid(response, &wm->last_processed_hash));
+				fmt_bitcoin_blkid(tmpctx, &wm->last_processed_hash));
 	return command_success(cmd, response);
 }
 
@@ -789,7 +789,7 @@ static struct command_result *json_getwatchmanheight(struct command *cmd,
 	json_add_u32(response, "height", height);
 	if (wm && wm->last_processed_height > 0)
 		json_add_string(response, "blockhash",
-				fmt_bitcoin_blkid(response, &wm->last_processed_hash));
+				fmt_bitcoin_blkid(tmpctx, &wm->last_processed_hash));
 	return command_success(cmd, response);
 }