Symbols: Don't include NoSymbols in the map

Symbols: Don't include NoSymbols in the map

Instead of using NoSymbol in the map, we use num_syms == 0 to signify
the non-presence of a symbol.  So instead of adding NoSymbol mappings
to the list regardless, detect them and set num_syms == 0.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>