add a memwipe function

It's meant to avoid being optimized out

memory_cleanse lifted from bitcoin

tests/unit_tests/memwipe.cpp

@@ -0,0 +1,64 @@
+// Copyright (c) 2017, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "gtest/gtest.h"
+
+#include <stdint.h>
+#include "misc_log_ex.h"
+#include "common/memwipe.h"
+
+// Probably won't catch the optimized out case, but at least we test
+// it works in the normal case
+static void test(bool wipe)
+{
+  char *foo = (char*)malloc(4);
+  ASSERT_TRUE(foo != NULL);
+  intptr_t foop = (intptr_t)foo;
+  strcpy(foo, "bar");
+  void *bar = wipe ? memwipe(foo, 3) : memset(foo, 0, 3);
+  ASSERT_EQ(foo, bar);
+  free(foo);
+  char *quux = (char*)malloc(4); // same size, just after free, so we're likely to get the same, depending on the allocator
+  if ((intptr_t)quux == foop)
+  {
+    MDEBUG(std::hex << std::setw(8) << std::setfill('0') << *(uint32_t*)quux);
+    if (wipe) ASSERT_TRUE(!memcmp(quux, "\0\0\0", 3));
+  }
+  else MWARNING("We did not get the same location, cannot check");
+  free(quux);
+}
+
+TEST(memwipe, control)
+{
+  test(false);
+}
+
+TEST(memwipe, works)
+{
+  test(true);
+}