Weird memory corruption

Question

This error only is occuring when I build with the WinDDK nmake compiler in 'free' (release), which performs optimizations. I can't reproduce this in the 'checked' build or compiling with VS.

Here is pseudo-code of what is happening in my code:

main()
{
   //variable init and other code
    fprintf(log, "pre nullValue: %lu\n", NULL);  //printf added for debugging
   otherFunc();
   funcWithError(str1, str2, NULL);
    fprintf(log, "post nullValue: %lu\n", NULL); 
    fprintf(log, "post2 nullValue: %lu, %lu, %lu\n", NULL, 0, NULL);
}

BOOL otherFunc()
{
   //variable init and other code
   callToDll();
    //...doing stuff
   libusb_usb_open(var1);    //If I remove this line there is no problem!!
    //...doing more stuff
}

BOOL funcWithError(char* s1, char* s2, FUNC_PTR fp)
{
    fprintf(log, "inFunc nullValue: %lu, %lu\n", NULL, fp);
   if(fp != NULL)
      return FALSE;        //This line is being executed erroneously!!
}

Output in log:
pre nullValue: 0
inFunc nullValue: 0, 251208
post nullValue: 251208
post2 nullValue: 251208, 251208, 251208
Note: The re-occurring number (251208) is a different number each time the program is run

Just changing that one line fixes/causes it. It is the libusb usb_open call.

1. Ultimately my question is to figure out how to fix the problem (I can't avoid that call)
2. But just on a stack/memory management level how is it even possible to have NULL not zero and have a literal value '0' print as non zero?

let me know what other information might help...