`MachOFile` link-edit readers force-unwrap missing `__LINKEDIT` data

[p-x9]

These initializers force-unwrap _fileSliceForLinkEditData / _readLinkEditData, which return nil when the range is not inside __LINKEDIT or the file:

Reader	Location
Symbols64 / Symbols	MachOFile+Symbols.swift:76, :81, :222, :227
ExportTrie	MachOFile+ExportTrie.swift:119
FunctionStarts	MachOFile+FunctionStarts.swift:37

Reproduce

• symbols64 on any MH_OBJECT (clang -c): relocatable objects have LC_SYMTAB but no __LINKEDIT segment.
• A truncated binary, or a symoff / stroff / dataoff pointing past EOF.

Suggestion

Return nil from symbols64 / symbols32 / exportTrie / functionStarts when the slice cannot be created. For objects, fall back to reading symoff / stroff relative to the file (they are plain file offsets there).