[core] fix crash when the style JSON doesn't have an object at its root

src/mbgl/style/parser.cpp

@@ -29,6 +29,11 @@ void Parser::parse(const std::string& json) {
         return;
     }
 
+    if (!document.IsObject()) {
+        Log::Error(Event::ParseStyle, "Style JSON must be an object");
+        return;
+    }
+
     if (document.HasMember("version")) {
         int version = document["version"].GetInt();
         if (version != 8) {

test/fixtures/style_parser/non-object.info.json

@@ -0,0 +1,7 @@
+{
+    "default": {
+        "log": [
+            [1, "ERROR", "ParseStyle", "Style JSON must be an object"]
+        ]
+    }
+}

test/fixtures/style_parser/non-object.style.json

@@ -0,0 +1 @@
+""