Ok, one huge problem apparently are these comparisons:

total_len > sizeof(buffer)

Since total_len is an int (and therefore signed), the behaviour is rather strange. I actually would have expected the compiler to prefer the type of the left side, but apparently it doesn't. So now we (sometimes) compare -1 > 8u which will then return true (since -1 will be cast into uint).

Replacing these four occurances with

total_len > (int)sizeof(buffer)

solves at least this particular problem. There is still some more memory corruption (since the next query fails at GetColumnInfo which, if executed alone, works fine). So I keep hunting ...