lightningd: fix log crash on weird escape lines from plugin.

Apparently clboss gives us \u UTF codes.  We don't support that (use UTF-8 directly)

```
126	../sysdeps/x86_64/multiarch/strlen-vec.S: No such file or directory.
(gdb) bt
    label=label@entry=0x63e2f9604db9 "char *[]") at ccan/ccan/tal/str/str.c:137
    complete=complete@entry=0x7ffe9090b0f6, destroyed=destroyed@entry=0x7ffe9090b0f7) at lightningd/plugin.c:773
```

Reported-by: Ken Sedgwick
Fixes: #8338
Changelog-None: broken in this release
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/plugin.c

@@ -522,6 +522,10 @@ static const char *plugin_log_handle(struct plugin *plugin,
 		const char *log_msg = json_escape_unescape(tmpctx, esc);
 		char **lines;
 
+		/* Weird \ escapes aren't handled by json_escape_unescape.  This is for you, clboss! */
+		if (!log_msg)
+			goto print_raw;
+
 		lines = tal_strsplit(tmpctx, log_msg, "\n", STR_EMPTY_OK);
 
 		for (size_t i = 0; lines[i]; i++) {
@@ -529,6 +533,7 @@ static const char *plugin_log_handle(struct plugin *plugin,
 			log_(plugin->log, level, NULL, call_notifier, "%s", lines[i]);
 		}
 	} else {
+	print_raw:
 		log_(plugin->log, level, NULL, call_notifier, "%.*s",
 		     msgtok->end - msgtok->start,
 		     plugin->buffer + msgtok->start);