[lld][macho] Fix segfault while processing malformed object file.

Author: Prabhuk

lld/MachO/InputFiles.cpp

@@ -808,6 +808,11 @@ void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
       continue;
 
     if ((sym.n_type & N_TYPE) == N_SECT) {
+      if (sym.n_sect == 0) {
+        error("Section symbol " + StringRef(strtab + sym.n_strx) + " in " +
+              toString(this) + " has an invalid section index of 0");
+        llvm_unreachable("Section symbol without an associated section.");
+      }
       Subsections &subsections = sections[sym.n_sect - 1]->subsections;
       // parseSections() may have chosen not to parse this section.
       if (subsections.empty())