fuzzers: add object parsing fuzzer

Add a simple fuzzer that exercises our object parser code. The fuzzer
is quite trivial in that it simply passes the input data directly to
`git_object__from_raw` for each of the four object types.

fuzzers/corpora/objects/commit

+tree 3e7ac388cadae960fe7e22175ce0da878afe9d18
+parent 8b89f362a34fcccdf1c6c5f3445895b71d9c6d56
+parent c590b41fe4057a84a9bd31a5605ceef2c309b0f8
+author Patrick Steinhardt <ps@pks.im> 1538760730 +0200
+committer GitHub <noreply@github.com> 1538760730 +0200
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+
+Merge pull request #4834 from pks-t/pks/v0.27.5
+
+Security release v0.27.5
\ No newline at end of file

fuzzers/corpora/objects/tag

+object a8d447f68076d1520f69649bb52629941be7031f
+type commit
+tag testtag
+tagger Patrick Steinhardt <ps@pks.im> 1539253015 +0200
+
+Tag message

fuzzers/objects_fuzzer.c

+/*
+ * libgit2 packfile fuzzer target.
+ *
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#include "git2.h"
+#include "object.h"
+
+#define UNUSED(x) (void)(x)
+
+int LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	UNUSED(argc);
+	UNUSED(argv);
+
+	if (git_libgit2_init() < 0)
+		abort();
+
+	return 0;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	const git_otype types[] = {
+		GIT_OBJ_BLOB, GIT_OBJ_TREE, GIT_OBJ_COMMIT, GIT_OBJ_TAG
+	};
+	git_object *object = NULL;
+	size_t i;
+
+	/*
+	 * Brute-force parse this as every object type. We want
+	 * to stress the parsing logic anyway, so this is fine
+	 * to do.
+	 */
+	for (i = 0; i < ARRAY_SIZE(types); i++) {
+		if (git_object__from_raw(&object, (const char *) data, size, types[i]) < 0)
+			continue;
+		git_object_free(object);
+		object = NULL;
+	}
+
+	return 0;
+}