Severity: bug

Steps:
1) emacs -Q from master branch (3adc1e7f379)
2) C-u M-& "ls"
   Error: shell-command: Wrong type argument: stringp, (4)

Cause:
When called interactively, `async-shell-command' pass argument
OUTPUT-BUFFER as `current-prefix-arg' to `shell-command'.  As per
docstring of `shell-command':

  "If OUTPUT-BUFFER is not a buffer and not nil (which happens
  interactively when the prefix argument is given), insert the output in
  current buffer after point leaving mark after it.  This cannot be done
  asynchronously."

Patch: