fix lib: Fix dx_diff_report_add string buffer realloc
fix lib: Fix dx_diff_report_add string buffer realloc

These changes reallocates the correct buffer after a truncated vsnprintf
string. They also optimize the reallocated size, and supply a copy of
the va_args to vsnprintf in order reuse it and avoid undefined behavior:

> As [vsnprintf] invoke the va_arg macro, the value of ap after the
> return is unspecified. [`man 3p vfprintf`]

veeg/disir-c,veeg/disir-c