lavf/webm_chunk: Fix a memory leak.

Fix a duplicate memory allocation. priv_data should be allocated
in line 64 call to avformat_alloc_output_context2 since we pass
the correct AVFormat to it. This removes the duplicate allocation.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
