util: systemd: Don't strlen a possibly NULL string

Coverity complains about virBufferCurrentContent might be return null
when calling strlen, so check virBufferError first before calling
strlen.

src/util/virsystemd.c

@@ -135,6 +135,8 @@ virSystemdAppendValidMachineName(virBufferPtr buf,
     bool skip_dot = false;
 
     for (; *name; name++) {
+        if (virBufferError(buf))
+            break;
         if (strlen(virBufferCurrentContent(buf)) >= 64)
             break;