Fix a couple of bugs uncovered by the Clang static analyzer

[haileys]

I ran the Clang static analyzer on a project I'm working on that uses hiredis and it found a couple of bugs.

There were three warnings, of which two are definitely legit. I have fixed these in the commits below.

The one warning I wasn't sure about was this one:

async.c:447:19: warning: The left operand of '!=' is a garbage value
        if (cb.fn != NULL) {
            ~~~~~ ^

This is in redisProcessCallbacks and it looks like it's triggerable when redisGetReply returns REDIS_OK but doesn't set reply to anything. I'm not familiar enough with the code to determine if this is possible though.

[pietern]

Looks good, thanks!

[pietern]

Btw, that cb is populated by this line:

hiredis/async.c

Line 419 in 070da21

if (__redisShiftCallback(&ac->replies,&cb) != REDIS_OK) {

[haileys]

@pietern Yep, although if the first call to redisGetReply doesn't read a full reply and we're in monitor mode, it looks like it may be possible that a garbage cb is pushed.