Verify file existence before deleting it

When creating a cached session file, a temporary file is generated; this
file is then renamed to the final cache file name. Afterwards, when
trying to erase this temporary file, a warning log entry is written
because the delete operation fails (the file does not exist anymore
giving it was renamed). Fix this by verifying a file exists before
trying to delete it.

Change-Id: If7e6435ccfbd5806a3167c6f07087a1082527628

src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java b/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java
index eefdd86..11e5b78 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCache.java
@@ -227,7 +227,7 @@
   }
 
   private void deleteFile(File f) {
-    if (!f.delete()) {
+    if (f.exists() && !f.delete()) {
       log.warn("Cannot delete file " + f.getName() + " from cache "
           + dir.getAbsolutePath());
     }