protobuf-c: parse_required_member(): zero bd->data in 0-length byte string case

in the case that we are unpacking a 0-length byte string, we need to
explicitly set bd->data to NULL, otherwise we may try to free() a stray
pointer when protobuf_c_message_free_unpacked() is called on the
containing message.

(Issue #157.)

protobuf-c/protobuf-c.c

@@ -2304,6 +2304,8 @@ parse_required_member(ScannedMember *scanned_member,
 			if (bd->data == NULL)
 				return FALSE;
 			memcpy(bd->data, data + pref_len, len - pref_len);
+		} else {
+			bd->data = NULL;
 		}
 		bd->len = len - pref_len;
 		return TRUE;