Fix UBSAN warnings in linux/device.c.

Fix UBSAN warnings in linux/device.c.

linux/device.c:149:11: runtime error: implicit conversion from type 'ssize_t' (aka 'long') of value -1 (64-bit, signed) to type 'size_t' (aka 'unsigned long') changed the value to 18446744073709551615 (64-bit, unsigned)
    #0 0x55e3cb851f84 in read_packet /home/runner/work/tinc/tinc/src/linux/device.c:149:11
    #1 0x55e3cb7bb7fe in handle_device_data /home/runner/work/tinc/tinc/src/net_packet.c:1906:5
    #2 0x55e3cb78e6e0 in event_loop /home/runner/work/tinc/tinc/src/event.c:353:5
    #3 0x55e3cb7a6a90 in main_loop /home/runner/work/tinc/tinc/src/net.c:505:6
    #4 0x55e3cb83d241 in main /home/runner/work/tinc/tinc/src/tincd.c:614:11
    #5 0x7fec881950b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #6 0x55e3cb757dcd in _start (/home/runner/work/tinc/tinc/src/tincd+0x9adcd)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior linux/device.c:149:11 in
linux/device.c:163:23: runtime error: unsigned integer overflow: 18446744073709551615 + 10 cannot be represented in type 'unsigned long'
    #0 0x55e3cb852253 in read_packet /home/runner/work/tinc/tinc/src/linux/device.c:163:23
    #1 0x55e3cb7bb7fe in handle_device_data /home/runner/work/tinc/tinc/src/net_packet.c:1906:5
    #2 0x55e3cb78e6e0 in event_loop /home/runner/work/tinc/tinc/src/event.c:353:5
    #3 0x55e3cb7a6a90 in main_loop /home/runner/work/tinc/tinc/src/net.c:505:6
    #4 0x55e3cb83d241 in main /home/runner/work/tinc/tinc/src/tincd.c:614:11
    #5 0x7fec881950b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #6 0x55e3cb757dcd in _start (/home/runner/work/tinc/tinc/src/tincd+0x9adcd)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior linux/device.c:163:23 in