tests/vhost-user-bridge: Fix misuse of isdigit()

vubr_set_host() passes char values to isdigit().  Undefined behavior
when the value is negative.

Fix by using qemu_isdigit() instead.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20190514180311.16028-3-armbru@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
[Missing #include "qemu-common.h" fixed]

tests/vhost-user-bridge.c

@@ -30,6 +30,7 @@
 #define _FILE_OFFSET_BITS 64
 
 #include "qemu/osdep.h"
+#include "qemu-common.h"
 #include "qemu/atomic.h"
 #include "qemu/iov.h"
 #include "standard-headers/linux/virtio_net.h"
@@ -645,7 +646,7 @@ vubr_host_notifier_setup(VubrDev *dev)
 static void
 vubr_set_host(struct sockaddr_in *saddr, const char *host)
 {
-    if (isdigit(host[0])) {
+    if (qemu_isdigit(host[0])) {
         if (!inet_aton(host, &saddr->sin_addr)) {
             fprintf(stderr, "inet_aton() failed.\n");
             exit(1);