patman: Only apply patches when we know the original HEAD

When patman applies the patches it checks out a new branch, uses 'git am'
to apply the patches one by one, and then tries to go back to the old
branch. If you try this when the branch is 'undefined', this doesn't work
as patman cannot restore the correct branch after applying the patches.
It seems that 'undefined' is created by git and is persistent after it is
created, so that you can end up on quite an old branch.

Add a check for the 'undefined' branch to avoid this.

Reported-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>