db: use correct backend for retrieving text fields.

Otherwise we don't have a nul terminator:

ep 22 02:25:42 lntxbot lightningd[804]: 2019/09/22 02:25:42 plugin-jqmethods Initialized jqmethods plugin.
Sep 22 02:30:01 lntxbot lightningd[804]: lightningd: FATAL SIGNAL 11 (version v0.7.2.1-255-g7686068)
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a0c6be send_backtrace
Sep 22 02:30:01 lntxbot lightningd[804]:         common/daemon.c:40
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a0c751 crashdump
Sep 22 02:30:01 lntxbot lightningd[804]:         common/daemon.c:53
Sep 22 02:30:01 lntxbot lightningd[804]: 0x7f1654d30f1f ???                                                                                                                                                                            Sep 22 02:30:01 lntxbot lightningd[804]:         ???:0
Sep 22 02:30:01 lntxbot lightningd[804]: 0x7f1654dad578 ???
Sep 22 02:30:01 lntxbot lightningd[804]:         ???:0
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a48b70 memcpy
Sep 22 02:30:01 lntxbot lightningd[804]:         /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a48b70 tal_dup_
Sep 22 02:30:01 lntxbot lightningd[804]:         ccan/ccan/tal/tal.c:796
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a46e81 tal_strdup_
Sep 22 02:30:01 lntxbot lightningd[804]:         ccan/ccan/tal/str/str.c:18
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a2e680 wallet_stmt2payment
Sep 22 02:30:01 lntxbot lightningd[804]:         wallet/wallet.c:2167
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a32711 wallet_payment_list
Sep 22 02:30:01 lntxbot lightningd[804]:         wallet/wallet.c:2446
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b039fb0d2 json_listsendpays
Sep 22 02:30:01 lntxbot lightningd[804]:         lightningd/pay.c:989
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b039f0d6f parse_request
Sep 22 02:30:01 lntxbot lightningd[804]:         lightningd/jsonrpc.c:650
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b039f1039 read_json
Sep 22 02:30:01 lntxbot lightningd[804]:         lightningd/jsonrpc.c:748
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a3ddee next_plan
Sep 22 02:30:01 lntxbot lightningd[804]:         ccan/ccan/io/io.c:59
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a3e2dd do_plan
Sep 22 02:30:01 lntxbot lightningd[804]:         ccan/ccan/io/io.c:407
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a3e30a io_ready
Sep 22 02:30:01 lntxbot lightningd[804]:         ccan/ccan/io/io.c:417
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b03a3fae2 io_loop
Sep 22 02:30:01 lntxbot lightningd[804]:         ccan/ccan/io/poll.c:445
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b039ee96c io_loop_with_timers
Sep 22 02:30:01 lntxbot lightningd[804]:         lightningd/io_loop_with_timers.c:24
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b039f2796 main
Sep 22 02:30:01 lntxbot lightningd[804]:         lightningd/lightningd.c:835
Sep 22 02:30:01 lntxbot lightningd[804]: 0x7f1654d13b96 ???
Sep 22 02:30:01 lntxbot lightningd[804]:         ???:0
Sep 22 02:30:01 lntxbot lightningd[804]: 0x561b039e1919 ???
Sep 22 02:30:01 lntxbot lightningd[804]:         ???:0
Sep 22 02:30:01 lntxbot lightningd[804]: 0xffffffffffffffff ???
Sep 22 02:30:01 lntxbot lightningd[804]:         ???:0

Reported-by: @fiatjaf
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

wallet/db.c

@@ -573,7 +573,7 @@ const void *db_column_blob(struct db_stmt *stmt, int col)
 
 const unsigned char *db_column_text(struct db_stmt *stmt, int col)
 {
-	return stmt->db->config->column_blob_fn(stmt, col);
+	return stmt->db->config->column_text_fn(stmt, col);
 }
 
 size_t db_count_changes(struct db_stmt *stmt)