flacdec: Fix mono output.

In the write callback of the flac decoder we correctly allocate and
fill an output buffer of n * 2 bytes for mono streams, where n is the
block size stored in the frame header. However, later in this function
twice as many bytes are added to the buffer tree. This may result in
a segmentation fault due to reading beyond the allocated buffer.

Fix this by adding n * channels * 2 bytes, which is correct for both
mono and stereo files.