hex: default to the_hash_algo on zero algorithm value

There are numerous places in the codebase where we assume we can
initialize data by zeroing all its bytes.  However, when we do that with
a struct object_id, it leaves the structure with a zero value for the
algorithm, which is invalid.

We could forbid this pattern and require that all struct object_id
instances be initialized using oidclr, but this seems burdensome and
it's unnatural to most C programmers.  Instead, if the algorithm is
zero, assume we wanted to use the default hash algorithm instead.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hex.c

@@ -124,6 +124,13 @@ char *hash_to_hex_algop_r(char *buffer, const unsigned char *hash,
 	char *buf = buffer;
 	int i;
 
+	/*
+	 * Our struct object_id has been memset to 0, so default to printing
+	 * using the default hash.
+	 */
+	if (algop == &hash_algos[0])
+		algop = the_hash_algo;
+
 	for (i = 0; i < algop->rawsz; i++) {
 		unsigned int val = *hash++;
 		*buf++ = hex[val >> 4];