Continue standardising malloc style for libcrypto

Continuing from previous commit ensure our style is consistent for malloc
return checks.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>

crypto/rand/rand_os2.c

@@ -149,7 +149,7 @@ int RAND_poll(void)
     if (DosQuerySysState) {
         char *buffer = OPENSSL_malloc(256 * 1024);
 
-        if (!buffer)
+        if (buffer == NULL)
             return 0;
 
         if (DosQuerySysState(0x1F, 0, 0, 0, buffer, 256 * 1024) == 0) {