util: git__memzero() tweaks

On Linux: fix a warning message related to the volatile qualifier (cast)
On Windows: use SecureZeroMemory()

On both, inline the call, so that no entry point can lead back to this "secure" memory zeroing.

src/util.c

@@ -722,12 +722,3 @@ void git__insertsort_r(
 	if (freeswap)
 		git__free(swapel);
 }
-
-void git__memzero(volatile void *data, size_t size)
-{
-	volatile uint8_t *scan = data;
-	uint8_t *end = scan + size;
-
-	while (scan < end)
-		*scan++ = 0x0;
-}

src/util.h

@@ -325,6 +325,16 @@ extern size_t git__unescape(char *str);
  * Safely zero-out memory, making sure that the compiler
  * doesn't optimize away the operation.
  */
-extern void git__memzero(volatile void *data, size_t size);
+GIT_INLINE(void) git__memzero(void *data, size_t size)
+{
+#ifdef _MSC_VER
+	SecureZeroMemory((PVOID)data, size);
+#else
+	volatile uint8_t *scan = (volatile uint8_t *)data;
+
+	while (size--)
+		*scan++ = 0x0;
+#endif
+}
 
 #endif /* INCLUDE_util_h__ */