fuzz: add basic fuzz testing target.

fuzz-pack-headers.c provides a fuzzing entry point compatible with
libFuzzer (and possibly other fuzzing engines).

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fuzz-pack-headers.c

@@ -0,0 +1,14 @@
+#include "packfile.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	enum object_type type;
+	unsigned long len;
+
+	unpack_object_header_buffer((const unsigned char *)data,
+				    (unsigned long)size, &type, &len);
+
+	return 0;
+}