ipadb_iterate(): handle match_entry == NULL

If match_entry == NULL all principals should be iterated.

Additionally this patch adds a check in ipadb_filter_escape() to make
sure that the input is not NULL.

Fixes: https://fedorahosted.org/freeipa/ticket/3011

daemons/ipa-kdb/ipa_kdb_common.c

@@ -30,6 +30,10 @@ char *ipadb_filter_escape(const char *input, bool star)
     size_t i = 0;
     size_t j = 0;
 
+    if (!input) {
+        return NULL;
+    }
+
     /* Assume the worst-case. */
     output = malloc(strlen(input) * 3 + 1);
     if (!output) {

daemons/ipa-kdb/ipa_kdb_principals.c

@@ -1879,6 +1879,12 @@ krb5_error_code ipadb_iterate(krb5_context kcontext,
         return KRB5_KDB_DBNOTINITED;
     }
 
+    /* If no match_entry is given iterate through all krb princs like the db2
+     * or ldap plugin */
+    if (match_entry == NULL) {
+        match_entry = "*";
+    }
+
     /* fetch list of principal matching filter */
     kerr = ipadb_fetch_principals(ipactx, 0, match_entry, &res);
     if (kerr != 0) {