Return a value from atomic read on Windows.

Use a read lock when reading using pthreads.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4517)

crypto/threads_pthread.c

@@ -177,7 +177,7 @@ int CRYPTO_atomic_read(int *val, int *ret, CRYPTO_RWLOCK *lock)
         return 1;
     }
 # endif
-    if (!CRYPTO_THREAD_write_lock(lock))
+    if (!CRYPTO_THREAD_read_lock(lock))
         return 0;
 
     *ret  = *val;

crypto/threads_win.c

@@ -135,7 +135,7 @@ int CRYPTO_atomic_add(int *val, int amount, int *ret, CRYPTO_RWLOCK *lock)
 
 int CRYPTO_atomic_read(int *val, int *ret, CRYPTO_RWLOCK *lock)
 {
-    InterlockedCompareExchange(val, 0, 0);
+    *ret = InterlockedCompareExchange(val, 0, 0);
     return 1;
 }