Only create backtrace stamp if dir exists.

If the build is aborted before the working directory is created,
there's also an induced error message about inability to create
a 'backtrace' file. But in that case, there is no subshells executing
yet.

Signed-off-by: Alexey Neyman <stilor@att.net>

scripts/functions

@@ -14,7 +14,7 @@ CT_OnError() {
     # To avoid printing the backtace for each sub-shell
     # up to the top-level, just remember we've dumped it
     if [ ! -f "${CT_WORK_DIR}/backtrace" ]; then
-        touch "${CT_WORK_DIR}/backtrace"
+        [ -d "${CT_WORK_DIR}" ] && touch "${CT_WORK_DIR}/backtrace"
 
         # Print steps backtrace
         step_depth=${CT_STEP_COUNT}