Work around language and compiler bug in memcpy, etc.

Most C standard library functions are undefined if passed NULL, even
when the corresponding length is zero. This gives them (and, in turn,
all functions which call them) surprising behavior on empty arrays.
Some compilers will miscompile code due to this rule. See also
https://www.imperialviolet.org/2016/06/26/nonnull.html

Add OPENSSL_memcpy, etc., wrappers which avoid this problem.

BUG=23

Change-Id: I95f42b23e92945af0e681264fffaf578e7f8465e
Reviewed-on: https://boringssl-review.googlesource.com/12928
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/ecdh/ecdh.c

diff --git a/crypto/ecdh/ecdh.c b/crypto/ecdh/ecdh.c
index 50a844e..22b216e 100644
--- a/crypto/ecdh/ecdh.c
+++ b/crypto/ecdh/ecdh.c
@@ -74,6 +74,8 @@
 #include <openssl/err.h>
 #include <openssl/mem.h>
 
+#include "../internal.h"
+
 
 int ECDH_compute_key(void *out, size_t outlen, const EC_POINT *pub_key,
                      const EC_KEY *priv_key,
@@ -140,7 +142,7 @@
     if (buflen < outlen) {
       outlen = buflen;
     }
-    memcpy(out, buf, outlen);
+    OPENSSL_memcpy(out, buf, outlen);
   }
 
   if (outlen > INT_MAX) {