Fix free with nil

Author: phimage

Sources/PatternPreferences.swift

@@ -492,12 +492,14 @@ open class KVCPreferences: PreferencesAdapter {
     }
 
     open func keys() -> [String] {
-        var names: [String] = []
         var count: UInt32 = 0
         // FIXME: not recursive?
-        let properties = class_copyPropertyList(self.object.classForCoder, &count)
+        guard let properties = class_copyPropertyList(self.object.classForCoder, &count) else {
+            return []
+        }
+        var names: [String] = []
         for i in 0 ..< Int(count) {
-            let property: objc_property_t = properties![i]
+            let property: objc_property_t = properties[i]
             let name: String = String(cString: property_getName(property))
             names.append(name)
         }