kex: NULL checks for 'first_kex_packet_follows'
kex: NULL checks for 'first_kex_packet_follows'

Add NULL checks to 'is_first_kex_packet_follows_guess_wrong'
to ensure that a 'strdup(NULL)' path can not be taken.

Signed-off-by: Jon Simons <44f878afe53efc66b76772bd845eb65944ed8232@jonsimons.org>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

nviennot/libssh,mwgoldsmith/libssh,kedazo/libssh,DouglasHeriot/libssh,wangshawn/libssh,elastichosts/libssh,kedazo/libssh,pouete/libssh,nviennot/libssh,kedazo/libssh,DouglasHeriot/libssh,kedazo/libssh,mwgoldsmith/libssh,Distrotech/libssh,mwgoldsmith/ssh,DouglasHeriot/libssh,DouglasHeriot/libssh,Distrotech/libssh,taikoo/libssh,robxu9/libssh,nviennot/libssh,wangshawn/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,elastichosts/libssh,mwgoldsmith/libssh,taikoo/libssh,robxu9/libssh,wangshawn/libssh,taikoo/libssh,elastichosts/libssh,Distrotech/libssh,nviennot/libssh,sebadoom/libssh,pouete/libssh,mwgoldsmith/ssh,taikoo/libssh,mwgoldsmith/ssh,sebadoom/libssh,sebadoom/libssh,robxu9/libssh,Distrotech/libssh,sebadoom/libssh,pouete/libssh,pouete/libssh,robxu9/libssh,wangshawn/libssh