db/postgres: avoid memleak.
```
"label": "db/db_postgres.c:294:char[]",
"backtrace": [
"ccan/ccan/tal/tal.c:442 (tal_alloc_)",
"ccan/ccan/tal/tal.c:471 (tal_alloc_arr_)",
"ccan/ccan/tal/str/str.c:91 (tal_vfmt_)",
"ccan/ccan/tal/str/str.c:44 (tal_fmt_)",
"db/db_postgres.c:294 (db_postgres_delete_columns)",
"wallet/db.c:1546 (migrate_payments_scids_as_integers)",
"wallet/db.c:986 (db_migrate)",
"wallet/db.c:1019 (db_setup)",
"wallet/wallet.c:101 (wallet_new)",
"lightningd/lightningd.c:1023 (main)",
"../csu/libc-start.c:308 (__libc_start_main)"
],
"parents": [
"db/utils.c:317:struct db",
"lightningd/lightningd.c:107:struct lightningd"
]
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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