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/senddata.c

@@ -6,7 +6,7 @@
 #define LIMIT 0x100000000UL
 
 int main(void) {
-  ssh_session session;
+  ssh_session session = NULL;
   ssh_channel channel;
   char buffer[1024*1024];
   int rc;
@@ -47,7 +47,7 @@ int main(void) {
     if(total > LIMIT)
       break;
   }
-    
+
   if (rc < 0) {
     printf("error : %s\n",ssh_get_error(session));
     ssh_channel_close(channel);