Fix an annoying double panic
Fix an annoying double panic

A double panic can screw up the first panic's stack trace.  Better not
to assert! anything when the thread is already panicing.

geofft/nix-rust,nix-rust/nix,geofft/nix-rust,geofft/nix-rust,carllerche/nix-rust,asomers/nix,carllerche/nix-rust,xd009642/nix,asomers/nix,MarkusJais/nix-rust,xd009642/nix,carllerche/nix-rust,MarkusJais/nix-rust,xd009642/nix,MarkusJais/nix-rust,nix-rust/nix