fix unreachable panic when parsing export table

[Orycterope]

Found this issue when fuzzing the crate.

The export symbol flag kind uses that last two bits of flags, and therefore can take 4 values. Only 3 are officially defined in the ExportSymbolKind enum.

Malformed macho files can define a symbol kind of 0b11 and will cause this crate to panic when parsing them.

Avoid panicking, and return a new error kind instead.