cleanup fixes for inf-ptrace.c

cleanup fixes for inf-ptrace.c

This is one of the stylistic patches.  The code here in inf-ptrace.c
is not incorrect, but it is in a style that the cleanup checker cannot
handle.  This patch changes it to a simpler style, following the usual
method of introducing an unconditional "outer" cleanup.

	* inf-ptrace.c (inf_ptrace_create_inferior): Unconditionally
	call discard_cleanups.
	(inf_ptrace_attach): Likewise.