[scan-build] fix warnings emitted on Clang Frontend code base

[apelete]

Summary

Fix "Logic error" warnings of the type "Called C++ object pointer is
null" reported by Clang Static Analyzer on the following file:

lib/Frontend/CompilerInstance.cpp.

Signed-off-by: Apelete Seketeli <[e-mail]>

lib/Frontend/CompilerInstance.cpp

assert(SourceMgr.getMainFileID().isValid() &&
            "Couldn't establish MainFileID!");
     return true;
   }
 
   StringRef InputFile = Input.getFile();
 
   // Figure out where to get and map in the main file.
   if (InputFile != "-") {
-    const FileEntry *File;
+    const FileEntry *File = nullptr;
     if (Opts.FindPchSource.empty()) {
       File = FileMgr.getFile(InputFile, /*OpenFile=*/true);
     } else {
       // When building a pch file in clang-cl mode, the .h file is built as if
       // it was included by a cc file.  Since the driver doesn't know about
       // all include search directories, the frontend must search the input
       // file through HeaderSearch here, as if it had been included by the
       // cc file at Opts.FindPchSource.
       const FileEntry *FindFile = FileMgr.getFile(Opts.FindPchSource);
       if (!FindFile) {
         Diags.Report(diag::err_fe_error_reading) << Opts.FindPchSource;
         return false;
       }
       const DirectoryLookup *UnusedCurDir;
       SmallVector<std::pair<const FileEntry *, const DirectoryEntry *>, 16>
           Includers;
       Includers.push_back(std::make_pair(FindFile, FindFile->getDir()));
+      if (HS)
         File = HS->LookupFile(InputFile, SourceLocation(), /*isAngled=*/false,
                               /*FromDir=*/nullptr,
                               /*CurDir=*/UnusedCurDir, Includers,
                               /*SearchPath=*/nullptr,
                               /*RelativePath=*/nullptr,
                               /*RequestingModule=*/nullptr,
                               /*SuggestedModule=*/nullptr, /*SkipCache=*/true);
       // Also add the header to /showIncludes output.
       if (File)
         DepOpts.ShowIncludesPretendHeader = File->getName();
     }
     if (!File) {
       Diags.Report(diag::err_fe_error_reading) << InputFile;
       return false;
     }