nixd/Parser: fix trailing-slash causing infinite loop in the Lexer #1359

Job	Run time
nix-build-develop-clang (none, release, true)	4m 8s
nix-build-develop-clang (none, release, false)	4m 1s
nix-build-develop-clang (none, debug, true)	4m 4s
nix-build-develop-clang (none, debug, false)	3m 49s
nix-build-develop-gcc (none, release, true)	6m 19s
nix-build-develop-gcc (none, release, false)	5m 43s
nix-build-develop-gcc (none, debug, true)	5m 14s
nix-build-develop-gcc (none, debug, false)	4m 41s
nix-build-develop-gcc (undefined, release, true)	9m 47s
nix-build-develop-gcc (undefined, release, false)	12m 49s
nix-build-develop-gcc (undefined, debug, true)	6m 35s
nix-build-develop-gcc (undefined, debug, false)	5m 21s
	1h 12m 31s

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