grep: prefer "struct grep_opt" over its "void *" equivalent

grep: prefer "struct grep_opt" over its "void *" equivalent

Stylistically fix up code added in bfac23d9534 (grep: Fix two memory
leaks, 2010-01-30). We usually don't use the "arg" at all once we've
casted it to the struct we want, let's not do that here when we're
freeing it. Perhaps it was thought that a cast to "void *" would
otherwise be needed?

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>