Check base property before using it

OverrideModule's `base` property should point a valid
module(OverridableModule).

It should be checked before using it.

Bug: 159503211
Test: add `override_apex` without `base`
      m emits error, not panic.
Change-Id: I1d7c9395cc9a7461e317dd4b6ddbec3a9e68e9bf
(cherry picked from commit 3851b0c1fe8e8ffd3d30b4b8a5289f39a2c98270)

android/override_module.go

@@ -223,6 +223,11 @@ var overrideBaseDepTag overrideBaseDependencyTag
 // next phase.
 func overrideModuleDepsMutator(ctx BottomUpMutatorContext) {
 	if module, ok := ctx.Module().(OverrideModule); ok {
+		base := String(module.getOverrideModuleProperties().Base)
+		if !ctx.OtherModuleExists(base) {
+			ctx.PropertyErrorf("base", "%q is not a valid module name", base)
+			return
+		}
 		// See if there's a prebuilt module that overrides this override module with prefer flag,
 		// in which case we call SkipInstall on the corresponding variant later.
 		ctx.VisitDirectDepsWithTag(PrebuiltDepTag, func(dep Module) {