virnetdevtap: Don't crash on !ifname in virNetDevTapInterfaceStats

https://bugzilla.redhat.com/show_bug.cgi?id=1595184

Some domain <interfaces/> do not have a name (because they are
not TAP devices). Therefore, if
virNetDevTapInterfaceStats(net->ifname, ...) is called an instant
crash occurs. In Linux version of the function strlen() is called
over the name and in BSD version STREQ() is called.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

src/util/virnetdevtap.c

@@ -691,6 +691,12 @@ virNetDevTapInterfaceStats(const char *ifname,
     FILE *fp;
     char line[256], *colon;
 
+    if (!ifname) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Interface name not provided"));
+        return -1;
+    }
+
     fp = fopen("/proc/net/dev", "r");
     if (!fp) {
         virReportSystemError(errno, "%s",
@@ -768,6 +774,12 @@ virNetDevTapInterfaceStats(const char *ifname,
     struct if_data *ifd;
     int ret = -1;
 
+    if (!ifname) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Interface name not provided"));
+        return -1;
+    }
+
     if (getifaddrs(&ifap) < 0) {
         virReportSystemError(errno, "%s",
                              _("Could not get interface list"));