Fix _nThreads lower bound to 0 (#83)

Fix problem with negative NTHREADS values

When a negative value is passed to NTHREADS, the value is read as int
and afterwards assigned to an unsigned int. During the assignment, the
negative value wraps around and becomes a ridiculously high number.
This can cause problems with TBB due to std::bad_alloc.

Now, the value is clipped to the nonnegative range before it is further
used.

src/libcadet/SimulatorImpl.cpp

@@ -1473,7 +1473,11 @@ namespace cadet
 		paramProvider.popScope();
 
 		if (paramProvider.exists("NTHREADS"))
-			_nThreads = paramProvider.getInt("NTHREADS");
+		{
+			// Ensure numThreads >= 0
+			const int numThreads = paramProvider.getInt("NTHREADS");
+			_nThreads = std::max(numThreads, 0);
+		}
 		else
 			_nThreads = 0;