libexpr: Fix read out-of-bound on the heap
libexpr: Fix read out-of-bound on the heap

Signed-off-by: Pamplemousse <e220f2b14804d9d261bf746623919194cf54cbd2@gmail.com>

NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix