CVE-2017-14632: vorbis_analysis_header_out: Don't clear opb if not initialized

If the number of channels is not within the allowed range
we call oggback_writeclear altough it's not initialized yet.

This fixes

    =23371== Invalid free() / delete / delete[] / realloc()
    ==23371==    at 0x4C2CE1B: free (vg_replace_malloc.c:530)
    ==23371==    by 0x829CA31: oggpack_writeclear (in /usr/lib/x86_64-linux-gnu/libogg.so.0.8.2)
    ==23371==    by 0x84B96EE: vorbis_analysis_headerout (info.c:652)
    ==23371==    by 0x9FBCBCC: ??? (in /usr/lib/x86_64-linux-gnu/sox/libsox_fmt_vorbis.so)
    ==23371==    by 0x4E524F1: ??? (in /usr/lib/x86_64-linux-gnu/libsox.so.2.0.1)
    ==23371==    by 0x4E52CCA: sox_open_write (in /usr/lib/x86_64-linux-gnu/libsox.so.2.0.1)
    ==23371==    by 0x10D82A: open_output_file (sox.c:1556)
    ==23371==    by 0x10D82A: process (sox.c:1753)
    ==23371==    by 0x10D82A: main (sox.c:3012)
    ==23371==  Address 0x68768c8 is 488 bytes inside a block of size 880 alloc'd
    ==23371==    at 0x4C2BB1F: malloc (vg_replace_malloc.c:298)
    ==23371==    by 0x4C2DE9F: realloc (vg_replace_malloc.c:785)
    ==23371==    by 0x4E545C2: lsx_realloc (in /usr/lib/x86_64-linux-gnu/libsox.so.2.0.1)
    ==23371==    by 0x9FBC9A0: ??? (in /usr/lib/x86_64-linux-gnu/sox/libsox_fmt_vorbis.so)
    ==23371==    by 0x4E524F1: ??? (in /usr/lib/x86_64-linux-gnu/libsox.so.2.0.1)
    ==23371==    by 0x4E52CCA: sox_open_write (in /usr/lib/x86_64-linux-gnu/libsox.so.2.0.1)
    ==23371==    by 0x10D82A: open_output_file (sox.c:1556)
    ==23371==    by 0x10D82A: process (sox.c:1753)
    ==23371==    by 0x10D82A: main (sox.c:3012)

as seen when using the testcase from CVE-2017-11333 with
008d23b782be09c8d75ba8190b1794abd66c7121 applied. However the error was
there before.