PrismFileLog: fix handling of close() call for stdout

If the underlying file handle corresponds to stdout, we ignore the
close() call. Correspondingly, we should not mark the log as closed,
as some of the functionality in PrismCL (e.g., for export to 'stdout'
in the explicit engine) relies on the possibility of closing the
corresponding log, leading to an exception. Bug was introduced during
recent efforts to make log closing more robust and catch
write-after-close use of logs.

Test case:
prism prism-examples/dice/dice.pm --exportmodel stdout.all -ex

prism/src/prism/PrismFileLog.java

@@ -146,7 +146,12 @@ public class PrismFileLog extends PrismLog
 			return;
 		}
 
-		if (!stdout) PrismNative.PN_CloseFile(fp);
+		if (stdout) {
+			// we never close stdout
+			return;
+		}
+
+		PrismNative.PN_CloseFile(fp);
 		// set fp to zero to indicate that the file handle is not valid anymore
 		fp = 0;
 	}