topology: builder: Fix possibly uninitialized variable in write_elem_block()

topology: builder: Fix possibly uninitialized variable in write_elem_block()

When an empty list is passed to write_elem_block(), it may leave
vendor_type uninitialized.
  builder.c: In function ‘write_elem_block’:
  builder.c:127:8: warning: ‘vendor_type’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    ret = write_block_header(tplg, tplg_type, vendor_type,
          ^
  builder.c:114:33: note: ‘vendor_type’ was declared here
    int ret, wsize = 0, count = 0, vendor_type;
                                   ^

Add an immediate return for count = 0 for avoiding it, and simplify
the code initializing vendor_type without using a one-shot loop.

Signed-off-by: Takashi Iwai <tiwai@suse.de>