Skip Bazel dist dir migration if not in dist mode or if Bazel dist dir does not exist.

Test: DIST_DIR=/tmp/foo USE_BAZEL=1 m dist and DIST_DIR=/tmp/foo USE_BAZEL=1 m dist installclean
Change-Id: Ibf576d2f3ba506e0f45c30a5dbea69de5c475c9a
diff --git a/cmd/soong_ui/main.go b/cmd/soong_ui/main.go
index bd1d450..74ede68 100644
--- a/cmd/soong_ui/main.go
+++ b/cmd/soong_ui/main.go
@@ -174,7 +174,7 @@
 
 	build.SetupOutDir(buildCtx, config)
 
-	if config.UseBazel() {
+	if config.UseBazel() && config.Dist() {
 		defer populateExternalDistDir(buildCtx, config)
 	}
 
@@ -547,6 +547,12 @@
 		return
 	}
 
+	// Make sure the internal DIST_DIR actually exists before trying to read from it
+	if _, err = os.Stat(internalDistDirPath); os.IsNotExist(err) {
+		ctx.Println("Skipping Bazel dist dir migration - nothing to do!")
+		return
+	}
+
 	// Make sure the external DIST_DIR actually exists before trying to write to it
 	if err = os.MkdirAll(externalDistDirPath, 0755); err != nil {
 		ctx.Fatalf("Unable to make directory %s: %s", externalDistDirPath, err)