extract_utils: Detect output existence before moving it for backup

* Avoids:
    Cleaning output directory (./../../../vendor/.../.../proprietary)..
    mv: cannot stat './../../../vendor/.../.../proprietary/*':
    No such file or directory

Change-Id: I3258e64f3997f139d8db4fb4e07d14964a469c78

build/tools/extract_utils.sh

@@ -806,7 +806,9 @@ function extract() {
         echo "Cleaning output directory ($OUTPUT_ROOT).."
         rm -rf "${OUTPUT_TMP:?}"
         mkdir -p "${OUTPUT_TMP:?}"
-        mv "${OUTPUT_ROOT:?}/"* "${OUTPUT_TMP:?}/"
+        if [ -d "$OUTPUT_ROOT" ]; then
+            mv "${OUTPUT_ROOT:?}/"* "${OUTPUT_TMP:?}/"
+        fi
         VENDOR_STATE=1
     fi