Fix UBSAN failure in b64decode_tinc()

Fix UBSAN failure in b64decode_tinc()

../src/utils.c:141:14: runtime error: implicit conversion from type 'int' of value -1 (32-bit, signed) to type 'unsigned int' changed the value to 4294967295 (32-bit, unsigned)
    #0 0x478d06 in b64decode_tinc /home/runner/work/tinc/tinc/openssl3/../src/utils.c:141:14
    #1 0x437f6c in dump_invitations /home/runner/work/tinc/tinc/openssl3/../src/tincctl.c:1116:6
    #2 0x42ebf6 in cmd_dump /home/runner/work/tinc/tinc/openssl3/../src/tincctl.c:1190:10
    #3 0x42b4c5 in run_command /home/runner/work/tinc/tinc/openssl3/../src/tincctl.c:3315:11
    #4 0x42aa24 in main /home/runner/work/tinc/tinc/openssl3/../src/tincctl.c:3366:15
    #5 0x7f6fb7a4c082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
    #6 0x406e2d in _start (/home/runner/work/tinc/tinc/openssl3/src/tinc+0x406e2d)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/utils.c:141:14 in