blame: handle broken commit headers gracefully

blame: handle broken commit headers gracefully

split_ident_line() can leave us with the pointers date_begin, date_end,
tz_begin and tz_end all set to NULL.  Check them before use and supply
the same fallback values as in the case of a negative return code from
split_ident_line().

The "(unknown)" is not actually shown in the output, though, because it
will be converted to a number (zero) eventually.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>