Add error check in redisContextInit

Check whether the obuf or reader is empty: if one of the items is empty,
free the redisContext.

hiredis.c

@@ -1059,6 +1059,12 @@ static redisContext *redisContextInit(void) {
     c->errstr[0] = '\0';
     c->obuf = sdsempty();
     c->reader = redisReaderCreate();
+
+    if (c->obuf == NULL || c->reader == NULL) {
+        redisFree(c);
+        return NULL;
+    }
+
     return c;
 }