iotests: Only kill NBD server if it runs

There may be NBD tests which do not create a sample image and simply
test whether wrong usage of the protocol is rejected as expected. In
this case, there will be no NBD server and trying to kill it during
clean-up will fail.

Cc: [email protected]
Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
(cherry picked from commit f798068)
Signed-off-by: Michael Roth <[email protected]>

Author: XanClic

tests/qemu-iotests/common.rc

@@ -189,7 +189,9 @@ _cleanup_test_img()
     case "$IMGPROTO" in
 
         nbd)
-            kill $QEMU_NBD_PID
+            if [ -n "$QEMU_NBD_PID" ]; then
+                kill $QEMU_NBD_PID
+            fi
             rm -f "$TEST_IMG_FILE"
             ;;
         file)