fix(libexpr-c): nix_get_external should use check_value_in, not check_value_out

nix_get_external reads from an already-initialized nix_value (set via
nix_init_external). Every other getter in the file (nix_get_bool,
nix_get_int, nix_get_float, nix_get_string, nix_get_path_string) uses
check_value_in, which asserts the value IS initialized.  nix_get_external
was the sole getter using check_value_out, which asserts the value is
UNinitialized and throws "nix_value already initialized" for any valid
external value — making the function always fail.

Author: Lillecarl

src/libexpr-c/nix_api_value.cc

@@ -335,7 +335,7 @@ ExternalValue * nix_get_external(nix_c_context * context, nix_value * value)
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        auto & v = check_value_out(value);
+        auto & v = check_value_in(value);
         assert(v.type() == nix::nExternal);
         return (ExternalValue *) v.external();
     }