Fix uninitialized digest_len field, causing WebSocket handshake to g_assert.

Fix uninitialized digest_len field, causing WebSocket handshake to g_assert.

g_checksum_get_digest checks to ensure that the passed digest_len is long
enough to hold the digest, before setting it to the actual length of the
digest returned.  Digest_len is uninitialized in the code, so if you're
lucky it will be larger than 20 and everything will work fine.  If you're
unlucky, g_checksum_get_digest will return either -1 or some number less
than 20, and the g_assert(digest_len==20) will fail.