Avoid assert when label is an empty string

Avoid assert when label is an empty string

Calling back() on an empty string is not allowed and triggers
an assert with recent gcc:

$ cd manual/PRESENTATION_Intro
$ ../../yosys counter.ys
...
/usr/include/c++/8/bits/basic_string.h:1136: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::back() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference = char&]: Assertion '!empty()' failed.

802             if (label.back() == ':' && GetSize(label) > 1)
(gdb) p label
$1 = ""