objdump fails to parse Mach-O binaries with n_desc bearing stabs

Summary:
The objdump Mach-O parser uses MachOObjectFile::checkSymbolTable() to
verify the symbol table is in a legal state before dereferencing the
offsets in the table. This routine missed a test for N_STAB symbols
when validating the two-level name space library ordinal for undefined
symbols. If the binary in question contained a value in the n_desc high
byte that is larger than the list of loaded dylibs, checkSymbolTable()
will flag the library ordinal as being out of range. Most of the time
the n_desc field is set to 0 or to small values, but old final linked
binaries exist with N_STAB symbols bearing non-trivial n_desc fields. 

The change here is simply to verify a symbol is not an N_STAB symbol
before consulting the values of n_other or n_desc.

rdar://44977336

Reviewers: lhames, pete, ab

Reviewed By: pete

Subscribers: llvm-commits, rupprecht

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58568

llvm-svn: 354722