Fail read calls after exception.

If a read call was performed after XrdFile threw an exception, it
is possible there are no active sources.  However, not all paths
check for the case of zero sources and may use-after-free the last
source.

This caused a deadlock at the Tier-0.

Utilities/XrdAdaptor/src/XrdRequestManager.cc

@@ -392,6 +392,17 @@ std::shared_ptr<XrdCl::File>
 RequestManager::getActiveFile()
 {
   std::lock_guard<std::recursive_mutex> sentry(m_source_mutex);
+  if (m_activeSources.empty())
+  {
+    edm::Exception ex(edm::errors::FileReadError);
+    ex << "XrdAdaptor::RequestManager::getActiveFile(name='" << m_name
+       << "', flags=0x" << std::hex << m_flags
+       << ", permissions=0" << std::oct << m_perms << std::dec
+       << ") => Source used after fatal exception.";
+    ex.addContext("In XrdAdaptor::RequestManager::handle()");
+    addConnections(ex);
+    throw ex;
+  }
   return m_activeSources[0]->getFileHandle();
 }
 
@@ -461,6 +472,17 @@ RequestManager::pickSingleSource()
             m_nextInitialSourceToggle = true;
         }
     }
+    else if (m_activeSources.empty())
+    {
+        edm::Exception ex(edm::errors::FileReadError);
+        ex << "XrdAdaptor::RequestManager::handle read(name='" << m_name
+               << "', flags=0x" << std::hex << m_flags
+               << ", permissions=0" << std::oct << m_perms << std::dec
+               << ") => Source used after fatal exception.";
+        ex.addContext("In XrdAdaptor::RequestManager::handle()");
+        addConnections(ex);
+        throw ex;
+    }
     else
     {
         source = m_activeSources[0];
@@ -569,14 +591,25 @@ XrdAdaptor::RequestManager::handle(std::shared_ptr<std::vector<IOPosBuffer> > io
     edm::CPUTimer timer;
     timer.start();
 
-    assert(m_activeSources.size());
     if (m_activeSources.size() == 1)
     {
         std::shared_ptr<XrdAdaptor::ClientRequest> c_ptr(new XrdAdaptor::ClientRequest(*this, iolist));
         checkSources(now, c_ptr->getSize());
         m_activeSources[0]->handle(c_ptr);
         return c_ptr->get_future();
     }
+    // Make sure active
+    else if (m_activeSources.empty())
+    {
+        edm::Exception ex(edm::errors::FileReadError);
+        ex << "XrdAdaptor::RequestManager::handle readv(name='" << m_name
+               << "', flags=0x" << std::hex << m_flags
+               << ", permissions=0" << std::oct << m_perms << std::dec
+               << ") => Source used after fatal exception.";
+        ex.addContext("In XrdAdaptor::RequestManager::handle()");
+        addConnections(ex);
+        throw ex;
+    }
 
     assert(iolist.get());
     std::shared_ptr<std::vector<IOPosBuffer> > req1(new std::vector<IOPosBuffer>);