[warm-reboot] fix warm-reboot when /tmp/cache is missing (#2367)

- What I did
Fixed issue when cache wasn't generated and warm reboot command fails.
Fixes sonic-net/sonic-buildimage#11914

- How I did it
Added a check for cache existence

- How to verify it
Run warm-reboot

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

scripts/fast-reboot

@@ -466,10 +466,13 @@ function save_counters_folder() {
         debug "Saving counters folder before warmboot..."
 
         counters_folder="/host/counters"
+        counters_cache="/tmp/cache"
         if [[ ! -d $counters_folder ]]; then
             mkdir $counters_folder
         fi
-        cp -rf /tmp/cache $counters_folder
+        if [[ -d $counters_cache ]]; then
+           cp -rf $counters_cache $counters_folder
+        fi
     fi
 }