Make llvm-ar behave like ar, if you create an empty archive, ar creates an empty archive.  llvm-ar would not generate an output file in this case

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47733 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Archive/ArchiveWriter.cpp b/lib/Archive/ArchiveWriter.cpp
index 07f7d88..04cc981 100644
--- a/lib/Archive/ArchiveWriter.cpp
+++ b/lib/Archive/ArchiveWriter.cpp
@@ -348,7 +348,7 @@
 {
   // Make sure they haven't opened up the file, not loaded it,
   // but are now trying to write it which would wipe out the file.
-  if (members.empty() && mapfile->size() > 8) {
+  if (members.empty() && mapfile && mapfile->size() > 8) {
     if (ErrMsg)
       *ErrMsg = "Can't write an archive not opened for writing";
     return true;