Fix potential undefined struct read
Fix potential undefined struct read

All the assignments to cb are inside conditionals, so it's
vaguely possible it never gets initialized before we try
to read from it with (cb.fn == NULL).

Condition discovered with scan-build.

Closes #229

hidebug/hiredis,chenlicong0821/hiredis,redis/hiredis,yiliaofan/hiredis,liulingfree/hiredis,rangan337/hiredis,thedrow/hiredis,galdor/hiredis,Yhgenomics/hiredis,h1048576/hiredis,tattsun/hiredis,liulingfree/hiredis,tattsun/hiredis,galdor/hiredis,xjzhou/hiredis,Microsoft/hiredis,redis/hiredis,arinal/hiredis,thomaslee/hiredis,jinguoli/hiredis,olgeni/hiredis,jinguoli/hiredis,charsyam/hiredis,olgeni/hiredis,owent-contrib/hiredis,Yhgenomics/hiredis,hidebug/hiredis,picrin/redisSamples,charsyam/hiredis,owent-contrib/hiredis,Microsoft/hiredis,redis/hiredis,jinguoli/hiredis,thomaslee/hiredis,badboy/hiredis-win,chenlicong0821/hiredis,jqk6/hiredis,rangan337/hiredis,xjzhou/hiredis,nokiddin/hiredis