x.py sometimes panics on Windows when the build is cancelled

[jyn514]

PS C:\Users\Joshua Nelson\src\rust> x check
Updating only changed submodules
Submodules updated in 0.14 seconds
   Compiling bootstrap v0.0.0 (C:\Users\Joshua Nelson\src\rust\src\bootstrap)
    Finished dev [unoptimized + debuginfo] target(s) in 13.28s

Build completed unsuccessfully in 0:00:14
PS C:\Users\Joshua Nelson\src\rust> thread 'main' panicked at 'PID `14496` doesn't seem to exist: The parameter is incorrect. (os error 87)', src\bootstrap\job.rs:106:5

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rust/src/bootstrap/job.rs

Lines 105 to 111 in e82b650

	let parent = OpenProcess(PROCESS_DUP_HANDLE, FALSE, pid.parse().unwrap());
	assert!(
	!parent.is_null(),
	"PID `{}` doesn't seem to exist: {}",
	pid,
	io::Error::last_os_error()
	);

[waterlens]

@rustbot claim