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/bn/bn_test.cc

diff --git a/crypto/bn/bn_test.cc b/crypto/bn/bn_test.cc
index 75ef17a..dfca2bc 100644
--- a/crypto/bn/bn_test.cc
+++ b/crypto/bn/bn_test.cc
@@ -680,7 +680,7 @@
 static bool TestBN2BinPadded(BN_CTX *ctx) {
   uint8_t zeros[256], out[256], reference[128];
 
-  memset(zeros, 0, sizeof(zeros));
+  OPENSSL_memset(zeros, 0, sizeof(zeros));
 
   // Test edge case at 0.
   bssl::UniquePtr<BIGNUM> n(BN_new());
@@ -689,13 +689,13 @@
             "BN_bn2bin_padded failed to encode 0 in an empty buffer.\n");
     return false;
   }
-  memset(out, -1, sizeof(out));
+  OPENSSL_memset(out, -1, sizeof(out));
   if (!BN_bn2bin_padded(out, sizeof(out), n.get())) {
     fprintf(stderr,
             "BN_bn2bin_padded failed to encode 0 in a non-empty buffer.\n");
     return false;
   }
-  if (memcmp(zeros, out, sizeof(out))) {
+  if (OPENSSL_memcmp(zeros, out, sizeof(out))) {
     fprintf(stderr, "BN_bn2bin_padded did not zero buffer.\n");
     return false;
   }
@@ -724,20 +724,21 @@
     }
     // Exactly right size should encode.
     if (!BN_bn2bin_padded(out, bytes, n.get()) ||
-        memcmp(out, reference, bytes) != 0) {
+        OPENSSL_memcmp(out, reference, bytes) != 0) {
       fprintf(stderr, "BN_bn2bin_padded gave a bad result.\n");
       return false;
     }
     // Pad up one byte extra.
     if (!BN_bn2bin_padded(out, bytes + 1, n.get()) ||
-        memcmp(out + 1, reference, bytes) || memcmp(out, zeros, 1)) {
+        OPENSSL_memcmp(out + 1, reference, bytes) ||
+        OPENSSL_memcmp(out, zeros, 1)) {
       fprintf(stderr, "BN_bn2bin_padded gave a bad result.\n");
       return false;
     }
     // Pad up to 256.
     if (!BN_bn2bin_padded(out, sizeof(out), n.get()) ||
-        memcmp(out + sizeof(out) - bytes, reference, bytes) ||
-        memcmp(out, zeros, sizeof(out) - bytes)) {
+        OPENSSL_memcmp(out + sizeof(out) - bytes, reference, bytes) ||
+        OPENSSL_memcmp(out, zeros, sizeof(out) - bytes)) {
       fprintf(stderr, "BN_bn2bin_padded gave a bad result.\n");
       return false;
     }
@@ -922,7 +923,7 @@
     }
 
     if (mpi_len != test.mpi_len ||
-        memcmp(test.mpi, scratch, mpi_len) != 0) {
+        OPENSSL_memcmp(test.mpi, scratch, mpi_len) != 0) {
       fprintf(stderr, "MPI test #%u failed:\n", (unsigned)i);
       hexdump(stderr, "Expected: ", test.mpi, test.mpi_len);
       hexdump(stderr, "Got:      ", scratch, mpi_len);
@@ -1062,7 +1063,8 @@
     }
     bssl::UniquePtr<uint8_t> delete_der(der);
     if (der_len != test.der_len ||
-        memcmp(der, reinterpret_cast<const uint8_t*>(test.der), der_len) != 0) {
+        OPENSSL_memcmp(der, reinterpret_cast<const uint8_t *>(test.der),
+                       der_len) != 0) {
       fprintf(stderr, "Bad serialization.\n");
       return false;
     }