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.

Also made the value parameter const to match all other getters.

Author: Lillecarl

src/libexpr-c/nix_api_value.cc

@@ -330,12 +330,12 @@ int64_t nix_get_int(nix_c_context * context, const nix_value * value)
     NIXC_CATCH_ERRS_RES(0);
 }
 
-ExternalValue * nix_get_external(nix_c_context * context, nix_value * value)
+ExternalValue * nix_get_external(nix_c_context * context, const 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();
     }

src/libexpr-c/nix_api_value.h

@@ -351,7 +351,7 @@ int64_t nix_get_int(nix_c_context * context, const nix_value * value);
  * @return reference valid while value is valid. Call nix_gc_incref() if you need it to live longer, then only in that
  * case call nix_gc_decref() when done. NULL in case of error
  */
-ExternalValue * nix_get_external(nix_c_context * context, nix_value * value);
+ExternalValue * nix_get_external(nix_c_context * context, const nix_value * value);
 
 /** @brief Get the ix'th element of a list
  * @ingroup value_extract