lib/Object/ELFObjectFile.cpp: Fix undefined behavior for MC/ELF/many-section.s not to fail (on msvc).

DenseMap::lookup(k) would return "default constructor value" when k was not met. It would be useless when value type were POD.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141774 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Object/ELFObjectFile.cpp b/lib/Object/ELFObjectFile.cpp
index a580dd0..7add2d8 100644
--- a/lib/Object/ELFObjectFile.cpp
+++ b/lib/Object/ELFObjectFile.cpp
@@ -444,8 +444,11 @@
 const typename ELFObjectFile<target_endianness, is64Bits>::Elf_Shdr *
 ELFObjectFile<target_endianness, is64Bits>
                              ::getSection(const Elf_Sym *symb) const {
-  if (symb->st_shndx == ELF::SHN_XINDEX)
+  if (symb->st_shndx == ELF::SHN_XINDEX) {
+    if (!ExtendedSymbolTable.count(symb))
+      return 0;
     return getSection(ExtendedSymbolTable.lookup(symb));
+  }
   if (symb->st_shndx >= ELF::SHN_LORESERVE)
     return 0;
   return getSection(symb->st_shndx);