[lld-macho][nfc]Add bounds on sections and subsections check before a…

…ttempting to dereferencing iterators.

Runnign some tests with asan built of LLD would throw errors similar to the following:

AddressSanitizer:DEADLYSIGNAL
    #0 0x55d8e6da5df7 in operator() /mnt/ssd/repo/lld/llvm-project/lld/MachO/Arch/ARM64.cpp:612
    #1 0x55d8e6daa514 in operator() /mnt/ssd/repo/lld/llvm-project/lld/MachO/Arch/ARM64.cpp:650

Differential Revision: https://reviews.llvm.org/D157027

lld/MachO/Arch/ARM64.cpp

@@ -602,11 +602,15 @@ void ARM64::applyOptimizationHints(uint8_t *outBuf, const ObjFile &obj) const {
         addr < sectionAddr + section->getSize())
       return true;
 
+    if (obj.sections.empty())
+      return false;
     auto secIt = std::prev(llvm::upper_bound(
         obj.sections, addr,
         [](uint64_t off, const Section *sec) { return off < sec->addr; }));
     const Section *sec = *secIt;
 
+    if (sec->subsections.empty())
+      return false;
     auto subsecIt = std::prev(llvm::upper_bound(
         sec->subsections, addr - sec->addr,
         [](uint64_t off, Subsection subsec) { return off < subsec.offset; }));