Allow DHCP server to process zero-length DHCP packet.

Process the received zero-length DHCP packet instead of closing
the listening socket and stopping reading packet, that will break
the follow-up DHCP handshake.

Bug: 269692093
Test: atest TetheringIntegrationTests
Change-Id: I2bff7518229ec96902da0c2e6368aeea55b2d8f9
1 file changed