Reported by Jeroen Ooms. On his particular Windows networking setup, when the C Driver makes a TXT query for the address specified in a mongodb+srv URI, it receives multiple responses that are not TXT records; I've observed A records in the results, maybe there are other record types. Apparently this is a behavior of the DnsQuery_UTF8 function we use on Windows. Since the C Driver assumes all responses to a TXT query are TXT records, and attempts to process them as TXT records, it reads an uninitialized field of the DNS_RECORD struct and crashes.
On Windows, the driver should check each record's type before trying to process it.