Exit early on genrule command parsing failure

cmd may be empty if android.Expand fails, and continuing results
in a panic.  Return immediately.

Test: m -j checkbuild
Change-Id: Ibf4d57ce048db9f7f16ce118ed9e9ecea5a6551c

genrule/genrule.go

@@ -198,6 +198,7 @@ func (g *generator) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 
 	if err != nil {
 		ctx.PropertyErrorf("cmd", "%s", err.Error())
+		return
 	}
 
 	ruleParams := blueprint.RuleParams{