compiler: don't double count "." in nested_function_num

compiler: don't double count "." in nested_function_num

    Nested functions are named "outerfunc.$nestedN", where N is a
    number. nested_function_num extracts that number. The name is
    first passed to unpack_hidden_name, which handles the "." and
    should result "$nestedN". Don't expect the "." again.

    This fixes assertion failure when escape analysis is enabled
    and -fgo-debug-escape is on. The failure looks

    go1: internal compiler error: in nested_function_num, at go/gofrontend/names.cc:241
    0x7bd7d3 Gogo::nested_function_num(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)

    Reviewed-on: https://go-review.googlesource.com/76213

From-SVN: r254504