avcodec/mjpegdec: postpone calling ff_get_buffer() until the SOS marker

avcodec/mjpegdec: postpone calling ff_get_buffer() until the SOS marker

With JPEG-LS PAL8 samples, the JPEG-LS extension parameters signaled with
the LSE marker show up after SOF but before SOS. For those, the pixel format
chosen by get_format() in SOF is GRAY8, and then replaced by PAL8 in LSE.
This has not been an issue given both pixel formats allocate the second data
plane for the palette, but after the upcoming soname bump, GRAY8 will no longer
do that. This will result in segfauls when ff_jpegls_decode_lse() attempts to
write the palette on a buffer originally allocated as a GRAY8 one.

Work around this by calling ff_get_buffer() after the actual pixel format is
known.

Signed-off-by: James Almer <jamrial@gmail.com>