Avoided calling __Builting_clz(0) with undefined behaviuor.

Check if argument is null before using this approach, and just return
zero when the value to do bit count on is zero.

Fixes a issue discovered by --enable-gcc-sanitizers in !31:

  ../../lib/encode.c:1384:49: runtime error: passing zero to clz(),
    which is not a valid argument
    #0 0x7f0af5dae97d in th_encode_ctl ../../lib/encode.c:1384
    #1 0x7f0af5e2eb97 in theora_encode_init ../../lib/encapiwrapper.c:72
    #2 0x55fd4beb18b7 in granulepos_test_encode
     ../../tests/granulepos_theora.c:71
    #3 0x55fd4beb126d in main ../../tests/granulepos_theora.c:137
    #4 0x7f0af51d3249 in __libc_start_call_main
     ../sysdeps/nptl/libc_start_call_main.h:58
    #5 0x7f0af51d3304 in __libc_start_main_impl ../csu/libc-start.c:360
    #6 0x55fd4beb1300 in _start
     (/home/user/libtheora/build/tests/.libs/granulepos_theoraenc+0x2300)

Fixes #2323