Add four, basic fuzz tests.

This change adds fuzzing tests for:
  ∙ Certificate parsing
  ∙ Private key parsing
  ∙ ClientHello parsing
  ∙ Server first flow (ServerHello, Certificate, etc) parsing.

Change-Id: I5f53282263eaaff69b1a03c819cca73750433653
Reviewed-on: https://boringssl-review.googlesource.com/6460
Reviewed-by: Adam Langley <agl@google.com>

fuzz/cert.cc

diff --git a/fuzz/cert.cc b/fuzz/cert.cc
new file mode 100644
index 0000000..ad40a31
--- /dev/null
+++ b/fuzz/cert.cc
@@ -0,0 +1,7 @@
+#include <openssl/x509.h>
+
+extern "C" int LLVMFuzzerTestOneInput(uint8_t *buf, size_t len) {
+  const uint8_t *bufp = buf;
+  X509_free(d2i_X509(NULL, &bufp, len));
+  return 0;
+}