lightningd: fix segfault when parse_filter fails

We need to initialize ->json_cmd *before* complaining about malformed
filters.

```
lightningd: FATAL SIGNAL 11 (version v25.12-21-g3851187-modded)
0x1042d2023 ???
        send_backtrace+0x4f:0
0x1042d20cb ???
        crashdump+0x43:0
0x19fe3b743 ???
        ???:0
0x104180173 command_log
        lightningd/jsonrpc.c:1406
0x10420d8f7 command_fail_badparam
        common/json_command.c:25
0x104181a07 parse_request
        lightningd/jsonrpc.c:1075
0x104181a07 read_json
        lightningd/jsonrpc.c:1216
0x10424c65b next_plan
        ccan/ccan/io/io.c:60
0x10424c65b do_plan
        ccan/ccan/io/io.c:422
0x10424c587 io_ready
        ccan/ccan/io/io.c:439
0x10424dd9b io_loop
        ccan/ccan/io/poll.c:470
0x10417ede7 io_loop_with_timers
        lightningd/io_loop_with_timers.c:22
0x104183a33 main
        lightningd/lightningd.c:1492
```

Co-authored-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: JSON-RPC: malformed filters no longer crash lightningd.

Author: dovgopoly

lightningd/jsonrpc.c

@@ -1070,6 +1070,13 @@ parse_request(struct json_connection *jcon,
 				    "Expected string for method");
 	}
 
+	c->json_cmd = find_cmd(jcon->ld->jsonrpc, buffer, method);
+	if (!c->json_cmd) {
+		return command_fail(
+		    c, JSONRPC2_METHOD_NOT_FOUND, "Unknown command '%.*s'",
+		    method->end - method->start, buffer + method->start);
+	}
+
 	if (filter) {
 		struct command_result *ret;
 		ret = parse_filter(c, "filter", buffer, filter);
@@ -1081,12 +1088,6 @@ parse_request(struct json_connection *jcon,
 	 * actually just logging the id */
 	log_io(jcon->log, LOG_IO_IN, NULL, c->id, NULL, 0);
 
-	c->json_cmd = find_cmd(jcon->ld->jsonrpc, buffer, method);
-	if (!c->json_cmd) {
-		return command_fail(
-		    c, JSONRPC2_METHOD_NOT_FOUND, "Unknown command '%.*s'",
-		    method->end - method->start, buffer + method->start);
-	}
 	if (!command_deprecated_in_ok(c, NULL,
 				      c->json_cmd->depr_start,
 				      c->json_cmd->depr_end)) {