Use 'event_ptid' instead of 'resume_ptid' on startup_inferior (fix for regression...

Use 'event_ptid' instead of 'resume_ptid' on startup_inferior (fix for regression on my last commit)

Pedro pointed out a regression happening on gdb.mi/mi-exec-run.exp,
and as it turned out, this was a thinko when dealing with some events
on startup_inferior.  Basically, one needs to pass 'event_ptid' to
target_mourn_inferior, but I mistakenly passed 'resume_ptid'.

This commit fixes it.

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gdb/ChangeLog:
2016-09-20  Sergio Durigan Junior  <sergiodj@redhat.com>

	* fork-inferior.c (startup_inferior): Pass 'event_ptid' instead of
	'resume_ptid' to 'target_mourn_inferior'.  Fix regression
	introduced by my last commit.