apply: clear_image() clears things a bit more

apply: clear_image() clears things a bit more

The clear_image() function did not clear the line table in the image
structure; this does not matter for the current callers, as the function
is only called from the codepaths that deal with binary patches where the
line table is never populated, and the codepaths that do populate the line
table free it themselves.

But it will start to matter when we introduce a codepath to retry a failed
patch, so make sure it clears and frees everything.

Signed-off-by: Junio C Hamano <gitster@pobox.com>