mythzmserver: Add a proper check for a failed call to mmap()

Should be checking for MAP_FAILED not NULL. Also add some additional debugging
showing why we failed to open the mmap file. Fixes #11187.

(cherry picked from commit 52b2773)

Signed-off-by: Paul Harrison <pharrison@mythtv.org>

mythplugins/mythzoneminder/mythzmserver/zmserver.cpp

@@ -1385,7 +1385,7 @@ void ZMServer::initMonitor(MONITOR *monitor)
 
         shm_ptr = mmap(NULL, shared_data_size, PROT_READ,
                                             MAP_SHARED, mapFile, 0x0);
-        if (shm_ptr == NULL)
+        if (shm_ptr == MAP_FAILED)
         {
             cout << "Failed to map shared memory from file [" << 
                 mmap_filename << "] " <<
@@ -1396,6 +1396,21 @@ void ZMServer::initMonitor(MONITOR *monitor)
             return;
         }
     }
+    else
+    {
+        // this is not necessarily a problem, maybe the user is still
+        // using the legacy shared memory support
+        if (m_debug)
+        {
+            cout << "Failed to open mmap file [" <<
+                mmap_filename << "] " <<
+                "for monitor: " <<
+                monitor->mon_id <<
+                " : " << strerror(errno) <<
+                endl;
+            cout << "Falling back to the legacy shared memory method" << endl;
+        }
+    }
 #endif
 
     if (shm_ptr == NULL)