patch 9.0.2120: un-used assignment in do_source_buffer_init

patch 9.0.2120: un-used assignment in do_source_buffer_init

Problem:  un-used assignment in do_source_buffer_init
Solution: Remove it

Coverity warns about assigning NULL to line in scriptfile.c:1408,
because right after that assignment, in the next iteration of the loop,
line will be overwritten by the next value from vim_strsave().

And in case this was the last iteration, the line variable is no longer
used until the function returns. So we can safely remove that assignment.

closes: #13547

Signed-off-by: Christian Brabandt <cb@256bit.org>