Also re-serialize X509 objects in fuzz/cert.cc.

This is a fairly common operation on an X509.

Change-Id: I1820f20b555f75c98ab7e3283b5530bc1c200e2a
Reviewed-on: https://boringssl-review.googlesource.com/7611
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

fuzz/cert.cc

@@ -1,11 +1,17 @@
+#include <openssl/mem.h>
 #include <openssl/x509.h>
 
 extern "C" int LLVMFuzzerTestOneInput(uint8_t *buf, size_t len) {
   const uint8_t *bufp = buf;
   X509 *x509 = d2i_X509(NULL, &bufp, len);
   if (x509 != NULL) {
-    /* Also extract the public key. */
+    /* Extract the public key. */
     EVP_PKEY_free(X509_get_pubkey(x509));
+
+    /* Reserialize the structure. */
+    uint8_t *der = NULL;
+    i2d_X509(x509, &der);
+    OPENSSL_free(der);
   }
   X509_free(x509);
   return 0;