remove trailing \x00

The trailing \x00 in the request header make the domain name as follow (domain\x00).
Which cause a bug when listing domains using qubesadminapi.
This bug occured when the icon-receiver is running on guivm other than dom0.

Author: qubest

window-icon-updater/icon-receiver

@@ -342,7 +342,7 @@ class IconReceiver(object):
         try:
             # Parse header from qrexec
             header = await reader.readuntil(b'\0')
-            header_parts = header.decode('ascii').split(' ')
+            header_parts = header.decode('ascii').split(' ').restrip('\x00')
             assert len(header_parts) >= 2, header_parts
 
             service_name = header_parts[0]