Fix segfault when required is not an array (#518)

See: sourcemeta/jsonschema#500
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/compiler/default_compiler_draft4.h

@@ -541,7 +541,9 @@ auto compiler_draft4_validation_required(const Context &context,
                                          const DynamicContext &dynamic_context,
                                          const Instructions &current)
     -> Instructions {
-  assert(schema_context.schema.at(dynamic_context.keyword).is_array());
+  if (!schema_context.schema.at(dynamic_context.keyword).is_array()) {
+    return {};
+  }
 
   if (schema_context.schema.defines("type") &&
       schema_context.schema.at("type").is_string() &&

test/evaluator/evaluator_draft4_test.cc

@@ -402,6 +402,29 @@ TEST(Evaluator_draft4, required_5_exhaustive) {
                                "The value was expected to be of type object");
 }
 
+TEST(Evaluator_draft4, required_6) {
+  // This is purposely invalid
+  const sourcemeta::core::JSON schema{sourcemeta::core::parse_json(R"JSON({
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "type": "object",
+    "properties": {
+      "foo": {
+        "required": "bar"
+      }
+    }
+  })JSON")};
+
+  const sourcemeta::core::JSON instance{sourcemeta::core::parse_json(R"JSON({
+    "foo": {}
+  })JSON")};
+
+  EVALUATE_WITH_TRACE_FAST_SUCCESS(schema, instance, 1);
+  EVALUATE_TRACE_PRE(0, AssertionTypeStrict, "/type", "#/type", "");
+  EVALUATE_TRACE_POST_SUCCESS(0, AssertionTypeStrict, "/type", "#/type", "");
+  EVALUATE_TRACE_POST_DESCRIBE(instance, 0,
+                               "The value was expected to be of type object");
+}
+
 TEST(Evaluator_draft4, allOf_1) {
   const sourcemeta::core::JSON schema{sourcemeta::core::parse_json(R"JSON({
     "$schema": "http://json-schema.org/draft-04/schema#",