add check for btn[i].ptr and d->lineButtons.instance[instance]

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4776 43c1dc8c-776e-4df2-8ef1-6e829299be21
fixes lineButtons.instance cleanup

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4777 43c1dc8c-776e-4df2-8ef1-6e829299be21