Fix error condition in vim integration when cursor is invalid.

[ahmedcharles]

Summary

This can happen, for example, if the file is empty.

[djasper]

Committed an alternative fix in r209417. Thanks for pointing this out!

tools/clang-format/clang-format.py

 style = 'file'
 
 def main():
   # Get the current text.
   buf = vim.current.buffer
   text = '\n'.join(buf)
 
   # Determine range to format.
-  cursor = int(vim.eval('line2byte(line("."))+col(".")')) - 2
+  cursor = int(vim.eval('line2byte(line("."))')) # line2byte is 1 based.
+  if cursor == -1: # -1 is an error, e.g. empty file
+    return
+  cursor = (cursor - 1) + vim.current.window.cursor[1] # col is 0 based.
   lines = '%s:%s' % (vim.current.range.start + 1, vim.current.range.end + 1)
 
   # Avoid flashing an ugly, ugly cmd prompt on Windows when invoking clang-format.
   startupinfo = None
   if sys.platform.startswith('win32'):
     startupinfo = subprocess.STARTUPINFO()
     startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
     startupinfo.wShowWindow = subprocess.SW_HIDE