Fix a crash in llvm-nm for a bad Mach-O file that has an N_SECT type symbol...

Fix a crash in llvm-nm for a bad Mach-O file that has an N_SECT type symbol and a zero n_sect value.

The code in llvm-nm for Mach-O files to determine the section type for an
N_SECT type symbol it will call getSymbolSection() and check for the error,
but in the case the n_sect value is zero it will return section_end() (aka nullptr).
And the code was using that and crashing instead of just returning a ’s’ for a
section or printing (?,?) as it would if getSymbolSection() returned an error.

rdar://33136604

llvm-svn: 313193