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/kex.c

@@ -341,7 +341,7 @@ static int cmp_first_kex_algo(const char *client_str,
     size_t client_kex_len;
     size_t server_kex_len;
 
-    char *colon;
+    char *colon = NULL;
 
     int is_wrong = 1;
 
@@ -779,7 +779,7 @@ char *ssh_client_select_hostkeys(ssh_session session)
 int ssh_set_client_kex(ssh_session session)
 {
     struct ssh_kex_struct *client = &session->next_crypto->client_kex;
-    const char *wanted;
+    const char *wanted = NULL;
     int ok;
     int i;