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/packet_crypt.c

@@ -262,7 +262,7 @@ int ssh_packet_hmac_verify(ssh_session session,
 {
     struct ssh_crypto_struct *crypto = NULL;
     unsigned char hmacbuf[DIGEST_MAX_LEN] = {0};
-    HMACCTX ctx;
+    HMACCTX ctx = NULL;
     size_t hmaclen = DIGEST_MAX_LEN;
     uint32_t seq;
     int cmp;