FFstrbuf: fix small FFstrbuf allocations in ffStrbufEnsureFree

[apocelipes]

Consider some corner cases:

• allocate was 28, need a new cap of 31 bytes. The old code grows the cap to 56 bytes. 24 bytes were wasted. (see below, you could waste more than 24 bytes)
• allocate was 8, need a new cap of 18 bytes. allocate was 8 -> 16 -> 32, no waste but multiplicated twice.
• allocate was 5, need a 11 bytes new cap. allocate was 5 -> 10 -> 20. You may think the new code will waste 12 bytes. However on the most of modern systems small memory areas are managed by blocks, blocks' sizes usually are powers of 2 or multiples of 16. Unfortunately, 20 is neither a power of 2 nor a multiple of 16, so it will be allocated into a 32-bytes block. Eventually there's no waste.

if 0 and 1 can be extended to 32 why 2, 3, 4, 5 cannot? It didn't hold water. Use FASTFETCH_STRBUF_DEFAULT_ALLOC as the smallest allocation size is more reasonable and more useful. Change if(allocate < 2) to if(allocate == 0) is resaonable but not useful.

In addition, the corner case actually exists in fastfetch -c all:

After fixing:

In other cases, memory usage did not change.