Fix test failure in 90-test_memleak.t

recent gcc versions can optimize the memory leak away,
avoid that by declaring the lost variable to be volatile.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #26269)

(cherry picked from commit eeb3266)

test/memleaktest.c

@@ -44,7 +44,7 @@ int main(int argc, char *argv[])
      */
     int exitcode = EXIT_FAILURE;
 #endif
-    char *lost;
+    char *volatile lost;
 
     lost = OPENSSL_malloc(3);
     if (!TEST_ptr(lost))