Fix product variable zero value check

The zero value check was being done by using reflect.DeepEqual on a
field from the default product variables, but this results in
comparison against a random type when the product variables struct
for the module has been filtered down.  Luckily this will always
fail false, which just removed and optimization but left the
behavior correct.

Use reflect.IsZero instead, which is both faster and correct.

Test: variable_test.go
Change-Id: Ieaaa590c2788ca39230e6695397e8ba8d1c6c103

android/module.go

@@ -539,7 +539,7 @@ func InitAndroidModule(m Module) {
 
 	// Allow tests to override the default product variables
 	if base.variableProperties == nil {
-		base.variableProperties = zeroProductVariables
+		base.variableProperties = defaultProductVariables
 	}
 
 	// Filter the product variables properties to the ones that exist on this module

android/variable.go

@@ -133,7 +133,7 @@ type variableProperties struct {
 	} `android:"arch_variant"`
 }
 
-var zeroProductVariables interface{} = variableProperties{}
+var defaultProductVariables interface{} = variableProperties{}
 
 type productVariables struct {
 	// Suffix to add to generated Makefiles
@@ -399,11 +399,9 @@ func variableMutator(mctx BottomUpMutatorContext) {
 	}
 
 	variableValues := reflect.ValueOf(a.variableProperties).Elem().FieldByName("Product_variables")
-	zeroValues := reflect.ValueOf(zeroProductVariables).FieldByName("Product_variables")
 
 	for i := 0; i < variableValues.NumField(); i++ {
 		variableValue := variableValues.Field(i)
-		zeroValue := zeroValues.Field(i)
 		name := variableValues.Type().Field(i).Name
 		property := "product_variables." + proptools.PropertyNameForField(name)
 
@@ -421,10 +419,9 @@ func variableMutator(mctx BottomUpMutatorContext) {
 		}
 
 		// Check if any properties were set for the module
-		if reflect.DeepEqual(variableValue.Interface(), zeroValue.Interface()) {
+		if variableValue.IsZero() {
 			continue
 		}
-
 		a.setVariableProperties(mctx, property, variableValue, val.Interface())
 	}
 }