Fix i2d_X509_AUX: pp can be NULL.

Reported by David Benjamin
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

crypto/x509/x_x509.c

@@ -184,14 +184,15 @@ X509 *d2i_X509_AUX(X509 **a, const unsigned char **pp, long length)
 int i2d_X509_AUX(X509 *a, unsigned char **pp)
 {
     int length, tmplen;
-    unsigned char *start = *pp;
+    unsigned char *start = pp != NULL ? *pp : NULL;
     length = i2d_X509(a, pp);
     if (length < 0 || a == NULL)
         return length;
 
     tmplen = i2d_X509_CERT_AUX(a->aux, pp);
     if (tmplen < 0) {
-        *pp = start;
+        if (start != NULL)
+            *pp = start;
         return tmplen;
     }
     length += tmplen;