linux-user: Check for bad event numbers in epoll_wait

The kernel checks that the maxevents parameter to epoll_wait
is non-negative and not larger than EP_MAX_EVENTS. Add this
check to our implementation, so that:
 * we fail these cases EINVAL rather than EFAULT
 * we don't pass negative or overflowing values to the
   lock_user() size calculation

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 21ae996..eecccbb 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -11501,6 +11501,11 @@
         int maxevents = arg3;
         int timeout = arg4;
 
+        if (maxevents <= 0 || maxevents > TARGET_EP_MAX_EVENTS) {
+            ret = -TARGET_EINVAL;
+            break;
+        }
+
         target_ep = lock_user(VERIFY_WRITE, arg2,
                               maxevents * sizeof(struct target_epoll_event), 1);
         if (!target_ep) {