fuzz-tests: Enhance b64_encode() validation with roundtrip decoding

Changelog-None: Currently, fuzz testing for b64_encode() merely
encodes input and frees the result, providing no real verification
of its behavior.

Introduce a new b64_decode() function (modeled after b32_decode())
and update the fuzz test to perform a roundtrip—encoding followed
by decoding—to ensure that b64_encode() correctly preserves the
original data.

common/base64.c

@@ -14,3 +14,12 @@ char *b64_encode(const tal_t *ctx, const void *data, size_t len)
 	str[enclen] = '\0';
 	return str;
 }
+
+u8 *b64_decode(const tal_t *ctx, const char *str, size_t len)
+{
+	size_t dlen = base64_decoded_length(len);
+	u8 *ret = tal_arr(ctx, u8, dlen);
+	if (base64_decode((char *)ret, dlen, str, len) < 0)
+		return tal_free(ret);
+	return ret;
+}

common/base64.h

@@ -5,5 +5,6 @@
 #include <ccan/tal/tal.h>
 
 char *b64_encode(const tal_t *ctx, const void *data, size_t len);
+u8 *b64_decode(const tal_t *ctx, const char *str, size_t len);
 
 #endif /* LIGHTNING_COMMON_BASE64_H */

tests/fuzz/fuzz-base32-64.c

@@ -21,5 +21,8 @@ void run(const uint8_t *data, size_t size)
 	tal_free(decoded);
 
 	encoded = b64_encode(NULL, data, size);
+	decoded = b64_decode(NULL, encoded, strlen(encoded));
+	assert(memcmp(decoded, data, size) == 0);
 	tal_free(encoded);
+	tal_free(decoded);
 }