build_command_info: free command_info on failure.

Once upon a time, command_info was a stack variable, now it is
dynamically allocated.  Coverity CID 299987.

plugins/sample/sample_plugin.c

@@ -239,6 +239,7 @@ oom:
     while (i > 0) {
 	free(command_info[i--]);
     }
+    free(command_info);
     return NULL;
 }