fix moving the cursor back up on failure

Fixes #9

When stdout is empty we were trying to move UP -1 lines, which
happens to work in some terminal emulators
but it turns out not in others, and is a silly thing to do anyway.

src/main.c

@@ -159,8 +159,9 @@ int read_command(const char *command, size_t *count, size_t *shown, size_t *tota
  waitpid(pid, &status, 0);
  if (status != 0) {
  // show the stderr instead
- termput1("UP", (*shown) - 1);
- *shown = *total = 0;
+ if ((*shown) > 0)
+ termput1("UP", *shown);
+ *count = *shown = *total = 0;
  read_show_output(c_stderr, count, shown, total);
  }
  fclose(c_stderr);