Fix closing driver library in context destructor

Loop over zeDrivers (instead of allDrivers) vector in order to free driver
libraries. Using allDrivers may not always work since if there's more than
one driver discovered, drivers in allDrivers vector have null handles.

Author: mateusztabaka

source/loader/ze_loader.cpp

@@ -824,7 +824,7 @@ namespace loader
             }
         }
 
-        for( auto& drv : allDrivers )
+        for( auto& drv : zeDrivers )
         {
             if (drv.handle) {
                 auto free_result = FREE_DRIVER_LIBRARY( drv.handle );