Prevent error message when formatting an empty file.

This fixes llvm.org/PR16514.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185531 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/clang-format/ClangFormat.cpp b/tools/clang-format/ClangFormat.cpp
index 33b7be9..2aa4325 100644
--- a/tools/clang-format/ClangFormat.cpp
+++ b/tools/clang-format/ClangFormat.cpp
@@ -162,6 +162,8 @@
     llvm::errs() << ec.message() << "\n";
     return true;
   }
+  if (Code->getBufferSize() == 0)
+    return true; // Empty files are formatted correctly.
   FileID ID = createInMemoryFile(FileName, Code.get(), Sources, Files);
   if (Offsets.empty())
     Offsets.push_back(0);