range-diff: don't segfault with mode-only changes

range-diff: don't segfault with mode-only changes

In ef283b3699 ("apply: make parse_git_diff_header public", 2019-07-11)
the 'parse_git_diff_header' function was made public and useable by
callers outside of apply.c.

However it was missed that its (then) only caller, 'find_header' did
some error handling, and completing 'struct patch' appropriately.

range-diff then started using this function, and tried to handle this
appropriately itself, but fell short in some cases.  This in turn
would lead to range-diff segfaulting when there are mode-only changes
in a range.

Move the error handling and completing of the struct into the
'parse_git_diff_header' function, so other callers can take advantage
of it.  This fixes the segfault in 'git range-diff'.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>