src/: Make line number overflows less likely

src/: Make line number overflows less likely

Huge files could trigger signed integer overflows if enough lines are
within the file. Use intmax_t which is at least 64 bit to move this
event far into the future.

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>