native/Linux: internal error if resume is short-circuited

native/Linux: internal error if resume is short-circuited

If the linux_nat_resume's short-circuits the resume because the
current thread has a pending status, and, a thread with a higher
number was previously stopped for a breakpoint, GDB internal errors,
like:

 /home/pedro/gdb/mygit/src/gdb/linux-nat.c:2590: internal-error: status_callback: Assertion `lp->status != 0' failed.

Fix this by make status_callback bail out earlier.  GDBserver is
already doing the same.

New test added that exercises this.

gdb/ChangeLog:
2015-03-19  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (status_callback): Return early if the LWP has no
	status pending.

gdb/testsuite/ChangeLog:
2015-03-19  Pedro Alves  <palves@redhat.com>

	* gdb.threads/continue-pending-status.c: New file.
	* gdb.threads/continue-pending-status.exp: New file.