lightningd: close plugin dir on return

Memory leak detected by ASan:

==880002==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 32816 byte(s) in 1 object(s) allocated from:
    #0 0x5039e7 in malloc (lightningd/lightningd+0x5039e7)
    #1 0x7f2e8c203884 in __alloc_dir (/lib64/libc.so.6+0xd2884)

lightningd/plugin.c

@@ -1703,9 +1703,11 @@ char *add_plugin_dir(struct plugins *plugins, const char *dir, bool error_ok)
 		} else {
 			p = plugin_register(plugins, fullpath, NULL, false,
 					    NULL, NULL);
-			if (!p && !error_ok)
+			if (!p && !error_ok) {
+				closedir(d);
 				return tal_fmt(NULL, "Failed to register %s: %s",
 				               fullpath, strerror(errno));
+			}
 		}
 	}
 	closedir(d);