Fix benign memory leak in uxpgnt.

No real need - when we fail to free this strbuf, we were about to exit
the whole process anyway - but it keeps Leak Sanitiser off my back, as
usual.

unix/uxpgnt.c

@@ -1191,6 +1191,7 @@ void run_agent(FILE *logfp, const char *symlink_path)
         exit(1);
     }
 
+    strbuf_free(upc->prompt_buf);
     conf_free(conf);
 }