Fix invoking "[kill|detach] inferiors" on inferiors that are not running

Invoking either of the above commands on an inferior that's not running
triggers the following assert failure:

  .../binutils-gdb/gdb/thread.c:514: internal-error: any_thread_of_process: Assertion `pid != 0' failed.

The fix is straightforward.  This patch also adds a test to check the
basic functionality of these commands, along with testing this fix in
particular.  Tested on x86_64 Linux.

gdb/ChangeLog:

	* inferior.c (detach_inferior_command): Don't call
	any_thread_of_process when pid is 0.
	(kill_inferior_command): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.base/kill-detach-inferiors-cmd.exp: New test file.
	* gdb.base/kill-detach-inferiors-cmd.c: New test file.