Fix crash when regenerating a patch with unquoted spaces in filename

[jorio]

This PR fixes a null pointer deref in the following scenario:

1. Write a patch that creates or deletes a file (with new file mode ... or delete file mode ...), and has unquoted filenames with spaces in the 1st line of the header.
   For example, take PATCH_ORIGINAL_NEW_FILE_WITH_SPACE in patch_common.h:

   diff --git a/sp ace.txt b/sp ace.txt
   new file mode 100644
   index 000000000..789819226
   --- /dev/null
   +++ b/sp ace.txt
   @@ -0,0 +1 @@
   +a

2. Get a git_diff from the patch with git_diff_from_buffer
3. Regenerate a patch with git_diff_to_buf

I'm not sure the proposed fix is optimal, since it doesn't reproduce the same diff --git header line. But, it at least prevents the crash, and vanilla git apply accepts the resulting patch.

[ethomson]

Thanks for the fix @jorio - I'm adding a minor formatting changes to match the libgit2 style a little bit better. I really appreciate the fix and the very nice tests.