nixd/Parser: fix trailing-slash causing infinite loop in the Lexer #1361

Job	Run time
nix-build-develop-gcc (none, release, true)	4m 46s
nix-build-develop-gcc (none, release, false)	6m 11s
nix-build-develop-gcc (none, debug, true)	4m 38s
nix-build-develop-gcc (none, debug, false)	4m 34s
nix-build-develop-gcc (undefined, release, true)	9m 2s
nix-build-develop-gcc (undefined, release, false)	10m 41s
nix-build-develop-gcc (undefined, debug, true)	5m 21s
nix-build-develop-gcc (undefined, debug, false)	6m 30s
nix-build-develop-clang (none, release, true)	4m 0s
nix-build-develop-clang (none, release, false)	4m 7s
nix-build-develop-clang (none, debug, true)	3m 40s
nix-build-develop-clang (none, debug, false)	4m 34s
	1h 8m 4s

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