util: buffer: Tolerate NULL 'buf' in virBufferStrcat

Most other buffer APIs tolerate the buffer being NULL.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

src/util/virbuffer.c

@@ -883,6 +883,9 @@ virBufferStrcat(virBufferPtr buf, ...)
 {
     va_list ap;
 
+    if (!buf)
+        return;
+
     va_start(ap, buf);
     virBufferStrcatVArgs(buf, ap);
     va_end(ap);