Fix QThread::idealThreadCount on Unix if sysconf or sysctl fails

The BSD4 code (including OS X) calls sysctl and if that fails, it sets
cores to -1. Similarly, the generic Unix code calls sysconf() and
assigns the returned value to cores, but sysconf can return -1 on
failure.

Change-Id: I9e521d366e9c42f36c2ba20a37e7a74539ddb8f4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/thread/qthread_unix.cpp

@@ -450,7 +450,8 @@ int QThread::idealThreadCount() Q_DECL_NOTHROW
     // the rest: Linux, Solaris, AIX, Tru64
     cores = (int)sysconf(_SC_NPROCESSORS_ONLN);
 #endif
-
+    if (cores == -1)
+        return 1;
     return cores;
 }