avcodec/wmavoice: set sframe_cache_size to the number of bits actually cached

copy_bits() silently writes nothing when the requested bits do not fit
the 256 byte superframe cache or exceed what is left in the packet, but
sframe_cache_size was set to the requested count regardless. On the next
packet the spillover bits were then appended at offset 0 and decoded as
a superframe of their own, or with no spillover, stale bytes from an
earlier packet were decoded again.

Use put_bits_count() so the size always matches the cache content. A
leftover larger than the cache is dropped instead, which is correct: a
speech superframe never needs more than about 1300 bits, so such a
leftover cannot be one. Bit reads never left sframe_cache, so this is
not a memory safety issue; the bogus superframes and the decoder errors
they produced on the affected samples are gone.

Using put_bits_count() for the size was suggested by the reporter.

Found-by: zhang xingxing <2388969553@qq.com>
Fixes: ZAKu08c7UMiZ