hw/ivshmem.c don't check for negative values on unsigned data types

There is no need to check for dest < 0 or vector >= 0 as both are
uint16_t.

This should fix problems with broken build with aggressive compiler
flags. Reported by Xudong Hao <xudong.hao@intel.com>

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Acked-by: Cam Macdonell <cam@cs.ualberta.ca>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit 1b27d7a1)

hw/ivshmem.c

@@ -199,13 +199,13 @@ static void ivshmem_io_writel(void *opaque, target_phys_addr_t addr,
 
         case DOORBELL:
             /* check that dest VM ID is reasonable */
-            if ((dest < 0) || (dest > s->max_peer)) {
+            if (dest > s->max_peer) {
                 IVSHMEM_DPRINTF("Invalid destination VM ID (%d)\n", dest);
                 break;
             }
 
             /* check doorbell range */
-            if ((vector >= 0) && (vector < s->peers[dest].nb_eventfds)) {
+            if (vector < s->peers[dest].nb_eventfds) {
                 IVSHMEM_DPRINTF("Writing %" PRId64 " to VM %d on vector %d\n",
                                                     write_one, dest, vector);
                 if (write(s->peers[dest].eventfds[vector],