command: avoid leak on failure

Detected by Coverity.  While it is possible on OOM condition, as
well as with bad code that passes binary == NULL, it is unlikely
to be encountered in the wild.

* src/util/command.c (virCommandNewArgList): Don't leak memory.

src/util/command.c

@@ -142,7 +142,7 @@ virCommandNewArgList(const char *binary, ...)
     const char *arg;
 
     if (!cmd || cmd->has_error)
-        return NULL;
+        return cmd;
 
     va_start(list, binary);
     while ((arg = va_arg(list, const char *)) != NULL)