[ELF] Disallow out-of-range section group indices after D70146

MaskRay · MaskRay · commit ce5de93e838f · 2019-11-19T09:49:45.000-08:00
Exposed by invalid/sht-group-wrong-section.test http://45.33.8.238/win/2613/step_9.txt
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
@@ -621,6 +621,8 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats) {
     InputSectionBase *head;
     InputSectionBase *prev = nullptr;
     for (uint32_t secIndex : entries.slice(1)) {
+      if (secIndex >= this->sections.size())
+        continue;
       InputSectionBase *s = this->sections[secIndex];
       if (!s || s == &InputSection::discarded)
         continue;
