pthread_conddestroy: Fix return value, if sem_getvalue fails

sem_getvalue returns ERROR and sets errno if it fails, we don't want to
return OK in this case, we want to return the non-negated error number.

libs/libc/pthread/pthread_conddestroy.c

@@ -72,7 +72,7 @@ int pthread_cond_destroy(FAR pthread_cond_t *cond)
       ret = sem_getvalue(&cond->sem, &sval);
       if (ret < 0)
         {
-          ret = -ret;
+          ret = get_errno();
         }
       else
         {