Clean up logic in memory_cleanse() for MSVC

Commit fbf327b13868861c2877c5754caf5a9816f2603c ("Minimal code
changes to allow msvc compilation.") was indeed minimal in terms
of lines touched. But as a result of that minimalism it changed the
logic in memory_cleanse() to first call std::memset() and then
additionally the MSVC-specific SecureZeroMemory() function, and it
also moved a comment to the wrong location.

This commit removes the superfluous call to std::memset() on MSVC
and ensures that the comment is in the right position again.

src/support/cleanse.cpp

@@ -30,14 +30,14 @@
  */
 void memory_cleanse(void *ptr, size_t len)
 {
+#if defined(_MSC_VER)
+    SecureZeroMemory(ptr, len);
+#else
     std::memset(ptr, 0, len);
 
     /* As best as we can tell, this is sufficient to break any optimisations that
        might try to eliminate "superfluous" memsets. If there's an easy way to
        detect memset_s, it would be better to use that. */
-#if defined(_MSC_VER)
-    SecureZeroMemory(ptr, len);
-#else
     __asm__ __volatile__("" : : "r"(ptr) : "memory");
 #endif
 }