Fix bug in banout_vprintf

[gpotter2]

Not really sure how nobody noticed, but at least on my

gcc version 14.2.0 (Debian 14.2.0-16)

$ ./bin/masscan --selftest
======================================================================
 Segmentation fault: please post this backtrace to:
 https://github.com/robertdavidgraham/masscan/issues
======================================================================
13: [./bin/masscan(+0xa871) [0x55e644345871]]
?? ??:0
[....]

marker is consumed when used in the first vnsprintf, resulting in a segfault in the second. See https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/va-arg-va-copy-va-end-va-start?view=msvc-170