Fix an undefined behavior when storing an empty StringRef.

Summary: Passing a nullptr to memcpy is UB.

Reviewers: ioeric

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D50966

llvm-svn: 340170

lib/Support/StringSaver.cpp

@@ -13,7 +13,8 @@ using namespace llvm;
 
 StringRef StringSaver::save(StringRef S) {
   char *P = Alloc.Allocate<char>(S.size() + 1);
-  memcpy(P, S.data(), S.size());
+  if (!S.empty())
+    memcpy(P, S.data(), S.size());
   P[S.size()] = '\0';
   return StringRef(P, S.size());
 }