clang-format: Add default fallback style.

Thanks to Michael Schlottke.

llvm-svn: 235162
diff --git a/clang/tools/clang-format/clang-format.py b/clang/tools/clang-format/clang-format.py
index b07160e..56a6e5d 100644
--- a/clang/tools/clang-format/clang-format.py
+++ b/clang/tools/clang-format/clang-format.py
@@ -34,6 +34,7 @@
 # a '.clang-format' or '_clang-format' file to indicate the style that should be
 # used.
 style = 'file'
+fallback_style = None
 if vim.eval('exists("g:clang_format_fallback_style")') == "1":
   fallback_style = vim.eval('g:clang_format_fallback_style')