test/fuzz: add fuzzing test for decoding decimals

NO_DOC=testing
NO_CHANGELOG=testing

(cherry picked from commit 4deadeb8)

test/fuzz/CMakeLists.txt

@@ -133,6 +133,13 @@ if (NOT ENABLE_UB_SANITIZER)
                    LIBRARIES xrow fuzzer_config)
 endif ()
 
+# Blocked by https://github.com/tarantool/tarantool/issues/8948.
+if (NOT ENABLE_UB_SANITIZER)
+  create_fuzz_test(PREFIX decimal_to_int64
+                   SOURCES decimal_to_int64_fuzzer.c
+                   LIBRARIES core fuzzer_config)
+endif ()
+
 include(ProtobufMutator)
 
 # UndefinedBehaviorSanitizer is not supported in LuaJIT.

test/fuzz/decimal_to_int64_fuzzer.c

+#include <string.h>
+#include <stdlib.h>
+#include "decimal.h"
+#include "trivia/util.h"
+
+int
+LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	char *buf = xcalloc(size + 1, sizeof(char));
+	if (!buf)
+		return 0;
+	memcpy(buf, data, size);
+	buf[size] = '\0';
+	decimal_t d;
+	decimal_from_string(&d, buf);
+	free(buf);
+
+	return 0;
+}