builtin-apply.c: get rid of an unnecessary use of temporary array

builtin-apply.c: get rid of an unnecessary use of temporary array

Instead of allocating a temporary array imglen[], copying contents to it
from another array img->line[], and then using imglen[], use the value
from img->line[], whose value does not change during the whole process.

This incidentally removes a use of C99 variable length array, which some
older compilers apparently are not happy with.

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