CVE-2025-4878 Initialize pointers where possible

This is mostly mechanical change initializing all the pointers I was able to
find with some grep and manual review of sources and examples.

Used the following greps (which yield some false positives though):

    git grep "    \w* *\* *\w*;$"
    git grep " ssh_session \w*;"
    git grep " ssh_channel \w*;"
    git grep " struct ssh_iterator \*\w*;"
    git grep " ssh_bind \w*;"
    git grep " ssh_key \w*;"
    git grep " ssh_string \w*;"
    git grep " ssh_buffer \w*;"
    git grep " HMACCTX \w*;"
    git grep " SHACTX \w*;"
    grep -rinP '^(?!.*=)\s*(?:\w+\s+)*\w+\s*\*\s*\w+\s*;'

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/ecdh_crypto.c

@@ -444,7 +444,7 @@ SSH_PACKET_CALLBACK(ssh_packet_server_ecdh_init)
     ssh_string q_c_string = NULL;
     ssh_string q_s_string = NULL;
     /* SSH host keys (rsa, ed25519 and ecdsa) */
-    ssh_key privkey;
+    ssh_key privkey = NULL;
     enum ssh_digest_e digest = SSH_DIGEST_AUTO;
     ssh_string sig_blob = NULL;
     ssh_string pubkey_blob = NULL;