fix potential mem corruption in utils_log_internal

[bratpiorka]

Fix potential mem corruption in utils_log_internal(), steps to reproduce:

tmp = snprintf(b_pos, b_size, "%s: ", func);
b_pos += (int)tmp;
b_size -= (int)tmp;

tmp = vsnprintf(b_pos, b_size, format, args);
b_pos += (int)tmp;
b_size -= (int)tmp;

The root cause is incorrect handling of the return values from snprintf() and vsnprintf(). These functions return the number of bytes that would have been written if enough space were available. The code unconditionally advances b_pos and reduces b_size by that return value, even when truncation occurred.

If the first snprintf() truncates, tmp can be larger than the remaining buffer. This moves b_pos past the end of the stack buffer and makes b_size invalid. The next vsnprintf() then writes through an invalid destination pointer, causing an out-of-bounds stack write.

[PatKamin]

Define b_size as size_t to avoid further casting of size_t to int in log_buffer_append(). I think sizeof should already return size_t type number.