Add empty pointer check in error cleanup

If realloc fails, the curargv will be NULL.

Closes #253

hiredis.c

@@ -895,11 +895,12 @@ int redisvFormatCommand(char **target, const char *format, va_list ap) {
     return totlen;
 
 err:
+    if (curargv) {
         while(argc--)
             sdsfree(curargv[argc]);
         free(curargv);
+    }
 
-    if (curarg != NULL)
     sdsfree(curarg);
 
     /* No need to check cmd since it is the last statement that can fail,