sequencer: don't die() on bogus user-edited timestamp

sequencer: don't die() on bogus user-edited timestamp

read_author_ident() is careful to handle errors "gently" when parsing
"rebase-merge/author-script" by printing a suitable warning and
returning NULL; it never die()'s. One possible reason that parsing might
fail is that "rebase-merge/author-script" has been hand-edited in such a
way which corrupts it or the information it contains.

However, read_author_ident() invokes fmt_ident() which is not so careful
about failing "gently". It will die() if it encounters a malformed
timestamp. Since read_author_ident() doesn't want to die() and since
it's dealing with possibly hand-edited data, take care to avoid passing
a bogus timestamp to fmt_ident().

A more "correctly engineered" fix would be to add a "gentle" version of
fmt_ident(), however, such a change it outside the scope of the bug-fix
series. If fmt_ident() ever does grow a "gentle" cousin, then the manual
timestamp check added here can be retired.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>