Fix a UBsan bot.

If we do not initialize Prefix here, Prefix.data() returns a nullptr.
Later, it is passed to memcpy. memcpy's behavior is undefined if src (or
dst) is a nullptr even if a given size is 0. That's why this code
triggered UBsan.

llvm-svn: 314368

lib/Support/TarWriter.cpp

@@ -127,6 +127,7 @@ static void writePaxHeader(raw_fd_ostream &OS, StringRef Path) {
 // Otherwise, returns false.
 static bool splitUstar(StringRef Path, StringRef &Prefix, StringRef &Name) {
   if (Path.size() < sizeof(UstarHeader::Name)) {
+    Prefix = "";
     Name = Path;
     return true;
   }