external: Fix a possible buffer overrun in bcrypt_pbkdf
external: Fix a possible buffer overrun in bcrypt_pbkdf

CID: #1250106

This fixes a 1 byte output overflow for large key length (not reachable
in libssh). Pulled from OpenBSD BCrypt PBKDF implementation.

Signed-off-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

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