Defensive asserts for RWLock

src/readwritelock.h

@@ -65,6 +65,7 @@ public:
     void releaseRead() {
         std::unique_lock<fastlock> rm(m_readLock);
         m_readCount--;
+        serverAssert(m_readCount >= 0);
         m_cv.notify_all();
     }
 
@@ -74,6 +75,7 @@ public:
         if (exclusive)
             m_writeLock.unlock();
         m_writeCount--;
+        serverAssert(m_writeCount >= 0);
         m_cv.notify_all();
     }