fuzz: test bolt12 bech32 decoding

tests/fuzz/Makefile

@@ -23,6 +23,7 @@ FUZZ_COMMON_OBJS := \
 	common/bigsize.o				\
 	common/blockheight_states.o			\
 	common/bolt11.o					\
+	common/bolt12_merkle.o				\
 	common/channel_config.o				\
 	common/close_tx.o				\
 	common/configdir.o				\
@@ -56,6 +57,7 @@ FUZZ_COMMON_OBJS := \
 	common/type_to_string.o				\
 	common/utils.o					\
 	common/version.o				\
+	wire/bolt12_wiregen.o				\
 	wire/fromwire.o					\
 	wire/onion_wiregen.o				\
 	wire/peer_wire.o				\

tests/fuzz/fuzz-bolt12-bech32-decode.c

@@ -0,0 +1,21 @@
+/* A fuzz target for the bolt12-specific bech32 decoding logic. */
+#include "config.h"
+#include <common/setup.h>
+#include <common/utils.h>
+#include <stddef.h>
+#include <tests/fuzz/libfuzz.h>
+
+/* Include bolt12.c directly, to gain access to string_to_data(). */
+#include "../../common/bolt12.c"
+
+void init(int *argc, char ***argv) { common_setup("fuzzer"); }
+
+void run(const u8 *data, size_t size)
+{
+	size_t dlen;
+	char *fail;
+
+	string_to_data(tmpctx, (const char *)data, size, "lno", &dlen, &fail);
+
+	clean_tmpctx();
+}