Fix buffer overflow when calling icecc-create-env

There are 1 program name + 3 arguments + the finishing NULL == 5
(+ extracount*2 arguments)
This could e.g. lead to "argv[pos++] = NULL" overwriting the contents of the
first strdup that follows "new char*[4+extracount*2]" (leading to
failing icecc-create-env call).