Fix pointer-to-non-volatile cast in thread_win.c.

Casting a pointer-to-non-volatile to pointer-to-volatile can be a no-op
as the compiler only requires volatile semantics when the pointed-to
object is a volatile object and there are no pointers-to-non-volatile
involved. This probably doesn't matter unless building with the MSVC
-volatile:iso flag, and maybe not even then, but it is good practice
anyway.

Change-Id: I94900d3dc61de3b8ce2ddecab2811907a9a7adbf
Reviewed-on: https://boringssl-review.googlesource.com/6973
Reviewed-by: David Benjamin <davidben@google.com>

crypto/internal.h

@@ -334,7 +334,7 @@ static inline int constant_time_select_int(unsigned int mask, int a, int b) {
 typedef uint32_t CRYPTO_once_t;
 #define CRYPTO_ONCE_INIT 0
 #elif defined(OPENSSL_WINDOWS)
-typedef LONG CRYPTO_once_t;
+typedef volatile LONG CRYPTO_once_t;
 #define CRYPTO_ONCE_INIT 0
 #else
 typedef pthread_once_t CRYPTO_once_t;

crypto/thread_win.c

@@ -31,9 +31,7 @@
 OPENSSL_COMPILE_ASSERT(sizeof(CRYPTO_MUTEX) >= sizeof(CRITICAL_SECTION),
                        CRYPTO_MUTEX_too_small);
 
-static void run_once(CRYPTO_once_t *in_once, void (*init)(void *), void *arg) {
-  volatile LONG *once = in_once;
-
+static void run_once(CRYPTO_once_t *once, void (*init)(void *), void *arg) {
   /* Values must be aligned. */
   assert((((uintptr_t) once) & 3) == 0);