Use intermediate vars to decrease minified size

Closure compiler applied to exports.visitModel shows a decrease
from 1005 bytes down to 930 bytes.

Author: timtucker

dist/knockout.mapping.js

@@ -780,10 +780,15 @@
                 if (ko.utils.arrayIndexOf(options.include, indexer) === -1) {
                     // The mapped properties object contains all the properties that were part of the original object.
                     // If a property does not exist, and it is not because it is part of an array (e.g. "myProp[3]"), then it should not be unmapped.
-                    if (unwrappedRootObject[mappingProperty] &&
-                        unwrappedRootObject[mappingProperty].mappedProperties && !unwrappedRootObject[mappingProperty].mappedProperties[indexer] &&
-                        unwrappedRootObject[mappingProperty].copiedProperties && !unwrappedRootObject[mappingProperty].copiedProperties[indexer] && (exports.getType(unwrappedRootObject) !== "array")) {
-                        return;
+                    var unwrappedRootMappingProperty = unwrappedRootObject[mappingProperty];
+                    if (unwrappedRootMappingProperty) {
+                        var mappedProperties = unwrappedRootMappingProperty.mappedProperties;
+                        if (mappedProperties && !mappedProperties[indexer]) {
+                            var copiedProperties = unwrappedRootMappingProperty.copiedProperties;
+                            if (copiedProperties && !copiedProperties[indexer] && (exports.getType(unwrappedRootObject) !== "array")) {
+                                return;
+                            }
+                        }
                     }
                 }
             }