Don't store blank data if we fail to get anything from the shared state file

Occasionally on some systems, the data returned from the shared state file is
bad/blank, despite locking. Ideally we'd find the cause of this and fix it,
but various attempts haven't identified exactly why this happens (including
running fsync after writing and before unlocking). If the data returned is
blank, we should use the latest available data rather than returning blank
data to the user/monitoring system.

lib/Resmon/Status.pm

@@ -42,9 +42,11 @@ sub get_shared_state {
             $blob = <$fh>;
         }
         flock($fh, LOCK_UN); # Release the lock
-        eval $blob;
-        die $@ if ($@);
-        $self->{store} = $VAR1;
+        if (defined $blob) {
+            eval $blob;
+            die $@ if ($@);
+            $self->{store} = $VAR1;
+        }
     } else {
         die "Unable to read shared state";
     };