grep: continue case insensitive fixed string search after NUL chars

Functions for C strings, like strcasestr(), can't see beyond NUL
characters.  Check if there is such an obstacle on the line and try
again behind it.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>