sftp: Fix a possible null pointer dereference in sftp_async_read.
sftp: Fix a possible null pointer dereference in sftp_async_read.

taikoo/libssh,nviennot/libssh,elastichosts/libssh,Distrotech/libssh,DouglasHeriot/libssh,mwgoldsmith/ssh,Distrotech/libssh,jt1/honeypot-libssh,Distrotech/libssh,taikoo/libssh,sebadoom/libssh,kedazo/libssh,robxu9/libssh,nviennot/libssh,mwgoldsmith/libssh,taikoo/libssh,nviennot/libssh,sebadoom/libssh,pouete/libssh,sebadoom/libssh,bigcat26/libssh-mod,robxu9/libssh,wangshawn/libssh,robxu9/libssh,pouete/libssh,mwgoldsmith/libssh,Distrotech/libssh,elastichosts/libssh,taikoo/libssh,DouglasHeriot/libssh,mwgoldsmith/libssh,wangshawn/libssh,kedazo/libssh,DouglasHeriot/libssh,kedazo/libssh,kedazo/libssh,sebadoom/libssh,robxu9/libssh,bigcat26/libssh-mod,mwgoldsmith/ssh,mwgoldsmith/libssh,wangshawn/libssh,elastichosts/libssh,jt1/honeypot-libssh,wangshawn/libssh,DouglasHeriot/libssh,bigcat26/libssh-mod,mwgoldsmith/ssh,pouete/libssh,nviennot/libssh,pouete/libssh,mwgoldsmith/ssh,jt1/honeypot-libssh,jt1/honeypot-libssh,bigcat26/libssh-mod