util/futex: replace double-cast check with a simple sign check

We want to know whether the signed int can be represented by an unsigned int
of the same size (no down-cast); for that, all we need is for it to be `>= 0`,
so let's check that.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28804>

src/util/futex.c

@@ -66,7 +66,7 @@ int futex_wait(uint32_t *addr, int32_t value, const struct timespec *timeout)
 
 int futex_wake(uint32_t *addr, int32_t count)
 {
-   assert(count == (int32_t)(uint32_t)count); /* Check that bits weren't discarded */
+   assert(count >= 0);
    return _umtx_op(addr, UMTX_OP_WAKE, (uint32_t)count, NULL, NULL) == -1 ? errno : 0;
 }