7013718: G1: small fixes for two assert/guarantee failures

Summary: Two small fixes to deal with a guarantee failure (the marking thread should join the SuspendibleThreadSet before calling a method that does pause prediction work so that said method is never called during a pause) and an assert failure (an assert is too strong).
Reviewed-by: iveresov, johnc

src/share/vm/gc_implementation/g1/concurrentMarkThread.cpp

@@ -251,7 +251,9 @@ void ConcurrentMarkThread::run() {
 
         // Now do the remainder of the cleanup operation.
         _cm->completeCleanup();
+        _sts.join();
         g1_policy->record_concurrent_mark_cleanup_completed();
+        _sts.leave();
 
         double cleanup_end_sec = os::elapsedTime();
         if (PrintGC) {

src/share/vm/gc_implementation/g1/heapRegionSeq.cpp

@@ -195,10 +195,10 @@ int HeapRegionSeq::find_contiguous(size_t num) {
     assert(0 <= res && res < _regions.length(),
            err_msg("res: %d should be valid", res));
     _alloc_search_start = res + (int) num;
-  }
     assert(0 < _alloc_search_start && _alloc_search_start <= _regions.length(),
            err_msg("_alloc_search_start: %d should be valid",
                    _alloc_search_start));
+  }
   return res;
 }