Guard against NULL spel, e.g. unnamed function parameters
Note tag along with .debug_info abbreviation code
Skip empty symbol tables when adding lexical blocks/variables

e.g. symbol tables for for(int i...) where there is no loop decl. This
prevents a zero-length lexical block (i.e. low_addr == high_addr),
which was tripping up debuggers.
for(int i = ...) scope example