Detect malformed LC_LINKER_COMMANDs in Mach-O binaries

Summary:
llvm-objdump can be tricked into reading beyond valid memory and
segfaulting if LC_LINKER_COMMAND strings are not null terminated. libObject
does have code to validate the integrity of the LC_LINKER_COMMAND struct,
but this validator improperly assumes linker command strings are null
terminated.

The solution is to report an error if a string extends beyond the end of
the LC_LINKER_COMMAND struct.

Reviewers: lhames, pete

Reviewed By: pete

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59179

llvm-svn: 355851