CTMC steady explicit: Fix regression introduced by #116

PR #116 (in particular 5c33a555ac) moved
the post-processing of the BSCC steady state probabilities (weighing
by the exit rates of the CTMC) into computeSteadyStateProbsForBSCC().

This introduced a regression, leading to wrong results, as the
post-processing would be applied on the solution vector that was
returned by the method, but not necessarily on the values passed back
out via the result argument vector.

We fix this by moving the post-processing up, before the copy and
adding a note of caution.

Failure e.g. for
prism prism-tests/papers/Par02/Par02-3.3.3.sm prism-tests/papers/Par02/Par02-3.3.3.sm.props -explicit -testall

prism/src/explicit/DTMCModelChecker.java

@@ -2553,6 +2553,11 @@ public class DTMCModelChecker extends ProbModelChecker
 			iterationsExport.close();
 		}
 
+		// Apply post processing on soln
+		if (bsccPostProcessor != null) {
+			bsccPostProcessor.apply(soln, states);
+		}
+
 		if (result != null && result != soln) {
 			// If result vector was passed in as method argument,
 			// it can be the case that result does not point to the current soln vector (most recent values)
@@ -2563,6 +2568,10 @@ public class DTMCModelChecker extends ProbModelChecker
 				result[state] = soln[state];
 			}
 		}
+		//
+		// NOTE: from here on, don't change the values of result/soln,
+		// as the values may have already been copied to the result vector above
+		//
 		// store only one result vector, free temporary vectors
 		result = soln;
 		soln = soln2 = null;
@@ -2574,11 +2583,6 @@ public class DTMCModelChecker extends ProbModelChecker
 			throw new PrismException(msg);
 		}
 
-		// Apply post processing
-		if (bsccPostProcessor != null) {
-			bsccPostProcessor.apply(result, states);
-		}
-
 		// Return results
 		ModelCheckerResult res = new ModelCheckerResult();
 		res.soln = result;