Remove use of exit()

[shakthimaan]

Since, hiredis is a client library that will be used by other applications, it shouldn't invoke exit.

It would be better to return an error or simply a non-zero value that the application can check for.

[pietern]

This is now available on master, for the blocking API (it should be OOM-safe now). The async API will fail on OOM. I'm not going to tackle that one, since it relies on other libraries that might abort on OOM, reducing the value of an OOM-safe async API.