OSThreads: Fix error code checking

pthread_join returns its error code and apparently doesn't set errno.

rts/posix/OSThreads.c

@@ -401,8 +401,9 @@ interruptOSThread (OSThreadId id)
 void
 joinOSThread (OSThreadId id)
 {
-    if (pthread_join(id, NULL) != 0) {
-        sysErrorBelch("joinOSThread: error %d", errno);
+    int ret = pthread_join(id, NULL);
+    if (ret != 0) {
+        sysErrorBelch("joinOSThread: error %d", ret);
     }
 }