[MachO] Fatal error on invalid section index

When there are more than 255 sections, MachO object writer allows
creation of object files which are potentially malformed. Currently,
there is an assertion in object writer code that prevents it. But for
distributions where assertions are turned on this still results in
creation of malformed objects. Turning the assertion into an explicit
fatal error.

Author: Prabhuk

llvm/lib/MC/MachObjectWriter.cpp

@@ -646,7 +646,8 @@ void MachObjectWriter::computeSymbolTable(
       LocalSymbolData.push_back(MSD);
     } else {
       MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
-      assert(MSD.SectionIndex && "Invalid section index!");
+      if (!MSD.SectionIndex)
+        report_fatal_error("Invalid section index!");
       LocalSymbolData.push_back(MSD);
     }
   }