Variable argument lists on x64

People have been reporting x64 builds of Stone Steps Webalizer crashing on Linux for about a year and even though I could see from the stack trace that the problem related to the variable argument list passed into vsnprintf, I couldn't figure out what exactly was going on because I don't have 64-bit hardware to reproduce this problem in a debugger.

The call stack always ended up in strlen called for a bad string with an invalid address, usually 0x3: