Avoid creating reference from nullptr.

[vitalybuka]

Summary

getFile() can return nullptr. This usually happens when Rels is empty so
the reference is never used. Still UBSAN complains.

lld/trunk/ELF/Relocations.cpp

static void scanRelocs(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels) {
   typedef typename ELFT::uint uintX_t;
 
   bool IsWrite = C.Flags & SHF_WRITE;
 
   auto AddDyn = [=](const DynamicReloc<ELFT> &Reloc) {
     Out<ELFT>::RelaDyn->addReloc(Reloc);
   };
 
-  const elf::ObjectFile<ELFT> &File = *C.getFile();
+  const elf::ObjectFile<ELFT> *File = C.getFile();
   ArrayRef<uint8_t> SectionData = C.Data;
   const uint8_t *Buf = SectionData.begin();
 
   ArrayRef<EhSectionPiece> Pieces;
   if (auto *Eh = dyn_cast<EhInputSection<ELFT>>(&C))
     Pieces = Eh->Pieces;
 
   ArrayRef<EhSectionPiece>::iterator PieceI = Pieces.begin();
   ArrayRef<EhSectionPiece>::iterator PieceE = Pieces.end();
 
   for (auto I = Rels.begin(), E = Rels.end(); I != E; ++I) {
     const RelTy &RI = *I;
-    SymbolBody &Body = File.getRelocTargetSym(RI);
+    SymbolBody &Body = File->getRelocTargetSym(RI);
     uint32_t Type = RI.getType(Config->Mips64EL);
 
     if (Config->MipsN32Abi) {
       uint32_t Processed;
       std::tie(Type, Processed) =
           mergeMipsN32RelTypes(Type, RI.r_offset, I + 1, E);
       I += Processed;
     }
 
     // We only report undefined symbols if they are referenced somewhere in the
     // code.
     if (!Body.isLocal() && Body.isUndefined() && !Body.symbol()->isWeak())
       reportUndefined(Body, C, RI.r_offset);
 
     RelExpr Expr = Target->getRelExpr(Type, Body);
     bool Preemptible = isPreemptible(Body, Type);
-    Expr = adjustExpr(File, Body, IsWrite, Expr, Type, Buf + RI.r_offset);
+    Expr = adjustExpr(*File, Body, IsWrite, Expr, Type, Buf + RI.r_offset);
     if (HasError)
       continue;
 
     // Skip a relocation that points to a dead piece
     // in a eh_frame section.
     while (PieceI != PieceE &&
            (PieceI->InputOff + PieceI->size() <= RI.r_offset))
       ++PieceI;
for (auto I = Rels.begin(), E = Rels.end(); I != E; ++I) {
     }
 
     // This relocation does not require got entry, but it is relative to got and
     // needs it to be created. Here we request for that.
     if (Expr == R_GOTONLY_PC || Expr == R_GOTONLY_PC_FROM_END ||
         Expr == R_GOTREL || Expr == R_GOTREL_FROM_END || Expr == R_PPC_TOC)
       In<ELFT>::Got->HasGotOffRel = true;
 
-    uintX_t Addend = computeAddend(File, Buf, E, RI, Expr, Body);
+    uintX_t Addend = computeAddend(*File, Buf, E, RI, Expr, Body);
 
     if (unsigned Processed =
             handleTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr)) {
       I += (Processed - 1);
       continue;
     }
 
     // Ignore "hint" and TLS Descriptor call relocation because they are
template <class ELFT> void scanRelocations(InputSectionBase<ELFT> &S) {
   if (S.AreRelocsRela)
     scanRelocs(S, S.relas());
   else
     scanRelocs(S, S.rels());
 }
 
 template <class ELFT, class RelTy>
 static void createThunks(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels) {
-  const elf::ObjectFile<ELFT> &File = *C.getFile();
+  const elf::ObjectFile<ELFT> *File = C.getFile();
   for (const RelTy &Rel : Rels) {
-    SymbolBody &Body = File.getRelocTargetSym(Rel);
+    SymbolBody &Body = File->getRelocTargetSym(Rel);
     uint32_t Type = Rel.getType(Config->Mips64EL);
     RelExpr Expr = Target->getRelExpr(Type, Body);
     if (!isPreemptible(Body, Type) && needsPlt(Expr))
       Expr = fromPlt(Expr);
-    Expr = Target->getThunkExpr(Expr, Type, File, Body);
+    Expr = Target->getThunkExpr(Expr, Type, *File, Body);
     // Some targets might require creation of thunks for relocations.
     // Now we support only MIPS which requires LA25 thunk to call PIC
     // code from non-PIC one, and ARM which requires interworking.
     if (Expr == R_THUNK_ABS || Expr == R_THUNK_PC || Expr == R_THUNK_PLT_PC) {
       auto *Sec = cast<InputSection<ELFT>>(&C);
       addThunk<ELFT>(Type, Body, *Sec);
     }
   }