Fix a typo in an error message for a check of invalid Mach-O files where

it was printing the field name fileoff instead of filesize.  The original check
was added in r278557.

This was found in tracking down the problem that lead to the fix in
r293842 - [dsymutil] Fix __LINKEDIT vmsize in dsymutil upgrade path

rdar://30386075

llvm-svn: 294354