[S] Change in flashrom[main]: erasure_layout: do not copy region buffers if they are null

[Anastasia Klimchuk (Code Review)]

Attention is currently required from: Aarya, Alexander Goncharov, Nikolai Artemiev.

Anastasia Klimchuk has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/81548?usp=email )

Change subject: erasure_layout: do not copy region buffers if they are null
......................................................................

Patch Set 1:

(1 comment)

File erasure_layout.c:

https://review.coreboot.org/c/flashrom/+/81548/comment/84d5dcf4_1a0fb98d :
PS1, Line 263: 	old_start_buf = (uint8_t *)malloc(old_start - region_start);
             : 	if (!old_start_buf) {
             : 		msg_cerr("Not enough memory!\n");
             : 		ret = -1;
             : 		goto _end;
             : 	}
             : 	old_end_buf = (uint8_t *)malloc(region_end - old_end);
             : 	if (!old_end_buf) {
             : 		msg_cerr("Not enough memory!\n");
             : 		ret = -1;
             : 		goto _end;
             : 	}
Oh I think this piece is also somewhat relevant to the patch. I remember looking at this code with suspicion few times, but I always forgot to follow up. Thank you for finding time and cleaning that up!

The suspicion I had was that `old_start_buf` and `old_end_buf` are allocated for potentially 0 size (because `old_start - region_start` and `region_end - old_end` are checked later). And when they are 0 size they are not needed!

However allocating for 0 size, while not having any practical purpose, probably prevented `memcpy` below (at the `_end` label) from failing, because malloc linux lan page says
> If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

So I think this allocation should go under if conditions 
`if (old_start - region_start)`
`if (region_end - old_end)`
because otherwise there is nothing to allocate

Do you think this can go in the same this patch?