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>

doc/guided_tour.dox

@@ -79,7 +79,7 @@ Here is a small example of how to use it:
 
 int main()
 {
-  ssh_session my_ssh_session;
+  ssh_session my_ssh_session = NULL;
   int verbosity = SSH_LOG_PROTOCOL;
   int port = 22;
 
@@ -126,7 +126,7 @@ Here's an example:
 
 int main()
 {
-  ssh_session my_ssh_session;
+  ssh_session my_ssh_session = NULL;
   int rc;
 
   my_ssh_session = ssh_new();
@@ -190,8 +190,8 @@ int verify_knownhost(ssh_session session)
     ssh_key srv_pubkey = NULL;
     size_t hlen;
     char buf[10];
-    char *hexa;
-    char *p;
+    char *hexa = NULL;
+    char *p = NULL;
     int cmp;
     int rc;
 
@@ -317,9 +317,9 @@ The example below shows an authentication with password:
 
 int main()
 {
-  ssh_session my_ssh_session;
+  ssh_session my_ssh_session = NULL;
   int rc;
-  char *password;
+  char *password = NULL;
 
   // Open session and set options
   my_ssh_session = ssh_new();
@@ -380,7 +380,7 @@ The example below shows how to execute a remote command:
 @code
 int show_remote_processes(ssh_session session)
 {
-  ssh_channel channel;
+  ssh_channel channel = NULL;
   int rc;
   char buffer[256];
   int nbytes;