Fix several issues in CLOG code:
* In `clg_str_reserve`, allocated memory may be bigger than requested
one, do not assign the latter back to `cstr->len_alloc`.
* `clg_str_vappendf` was mis-interpreting returned value from
`vsnprintf`, and completely mixing total allocated memory and extra
needed amount of memory to allocate...
Simplified code of `clg_str_vappendf` to only have allocating code
handled in one place, makes things easier to follow too.
Think this should also be beckported to 2.83.
9cac158e96