scripts/gen-lockedsig-cache: Don't list paths which don't exist

This avoids failures seen on the autobuilder when generating eSDKs
and release sstate copies.

(From OE-Core rev: 8ffe6ca984e034976322beea9b16c92d46708d15)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/gen-lockedsig-cache

@@ -24,6 +24,8 @@ def extract_sha(filename):
 # a map from hash to list of file with that hash
 def map_sha_to_files(dir_, prefix, sha_map):
     sstate_prefix_path = dir_ + '/' + prefix + '/'
+    if not os.path.exists(sstate_prefix_path):
+        return
     sstate_files = os.listdir(sstate_prefix_path)
     for f in sstate_files:
         try: