fuzz: add a fuzz target for common/descriptor_checksum

A small one just to check that we don't crash nor go out of bounds!

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tests/fuzz/Makefile

@@ -21,6 +21,7 @@ FUZZ_COMMON_OBJS := \
 	common/daemon.o					\
 	common/daemon_conn.o				\
 	common/derive_basepoints.o			\
+	common/descriptor_checksum.o			\
 	common/fee_states.o				\
 	common/htlc_state.o				\
 	common/permute_tx.o				\

tests/fuzz/fuzz-descriptor_checksum.c

@@ -0,0 +1,20 @@
+#include <tests/fuzz/libfuzz.h>
+
+#include <ccan/tal/tal.h>
+#include <common/descriptor_checksum.h>
+
+void init(int *argc, char ***argv)
+{
+}
+
+void run(const uint8_t *data, size_t size)
+{
+	char *string;
+	struct descriptor_checksum checksum;
+
+	/* We should not crash nor overflow the checksum buffer. */
+
+	string = to_string(NULL, data, size);
+	descriptor_checksum(string, tal_count(string), &checksum);
+	tal_free(string);
+}