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.

Index: krb5-1.10.2/src/lib/krb5/krb/princ_comp.c
===================================================================
--- krb5-1.10.2.orig/src/lib/krb5/krb/princ_comp.c
+++ krb5-1.10.2/src/lib/krb5/krb/princ_comp.c
@@ -36,6 +36,12 @@ realm_compare_flags(krb5_context context
     const krb5_data *realm1 = krb5_princ_realm(context, princ1);
     const krb5_data *realm2 = krb5_princ_realm(context, princ2);
 
+    if ((princ1 == NULL) || (princ2 == NULL))
+        return FALSE;
+
+    if ((realm1 == NULL) || (realm2 == NULL))
+        return FALSE;
+
     if (realm1->length != realm2->length)
         return FALSE;
 
@@ -87,6 +93,9 @@ 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 (krb5_princ_type(context, princ1) == KRB5_NT_ENTERPRISE_PRINCIPAL) {