Reference docs don't define what happens if you call krb5_realm_compare() with
malformed krb5_principal structures.  Define a behavior which keeps it from
crashing if applications don't check ahead of time.

--- krb5/src/lib/krb5/krb/princ_comp.c
+++ krb5/src/lib/krb5/krb/princ_comp.c
@@ -41,6 +41,10 @@ realm_compare_flags(krb5_context context
     const krb5_data *realm1 = &princ1->realm;
     const krb5_data *realm2 = &princ2->realm;
 
+    if (princ1 == NULL || princ2 == NULL)
+        return FALSE;
+    if (realm1 == NULL || realm2 == NULL)
+        return FALSE;
     if (realm1->length != realm2->length)
         return FALSE;
     if (realm1->length == 0)
@@ -92,6 +98,9 @@ krb5_principal_compare_flags(krb5_contex
     krb5_principal upn2 = NULL;
     krb5_boolean ret = FALSE;
 
+    if (princ1 == NULL || princ2 == NULL)
+        return FALSE;
+
     if (flags & KRB5_PRINCIPAL_COMPARE_ENTERPRISE) {
         /* Treat UPNs as if they were real principals */
         if (princ1->type == KRB5_NT_ENTERPRISE_PRINCIPAL) {