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>

examples/scp_download.c

@@ -113,7 +113,7 @@ static int fetch_files(ssh_session session)
     int size;
     char buffer[BUF_SIZE];
     int mode;
-    char *filename;
+    char *filename = NULL;
     int r;
     ssh_scp scp = ssh_scp_new(session,
                               SSH_SCP_READ | SSH_SCP_RECURSIVE,
@@ -179,7 +179,7 @@ end:
 
 int main(int argc, char **argv)
 {
-    ssh_session session;
+    ssh_session session = NULL;
     if (opts(argc, argv) < 0)
         return EXIT_FAILURE;
     session = connect_ssh(host, NULL, verbosity);