Also fixes a bug in UnDecorateSymbolName when undecorated_length == 0.

Images in the shared cache have their segments mapped non-contiguously.  We
don't know how to find where the non-__TEXT segments actually are.  Also, the
code assumes that the image is mapped contiguously in various places.  So, we
just ignore those segments.

Treating the non-contiguous segments as though they were contiguous caused
their apparent memory ranges to overlap, leading to addresses being attributed
to the wrong module, let alone symbol.